diff --git a/readme.md b/readme.md
index f634ff9..5b94346 100644
--- a/readme.md
+++ b/readme.md
@@ -20,6 +20,29 @@ Add the mailer service to the classpath by adding the following maven dependency
 If you want to use attachments, you will have to create a bean implementing this interface: [AttachmentDataSource.java](src%2Fmain%2Fjava%2Fit%2Faboutbits%2Fspringboot%2Femailservice%2Flib%2FAttachmentDataSource.java)  
 This step is optional.
 
+#### Inline (CID) attachments
+
+To embed an attachment inline, set a `contentId` on the attachment and reference it in the `htmlBody` via the `cid:` scheme.
+This is the equivalent of `MimeMessageHelper.addInline(...)` and renders in all major email clients.
+
+```java
+// @formatter:off
+EmailParameter.Email.builder()
+        // ...
+        .htmlBody("<img src=\"cid:header-logo\"><h1>Hello!</h1>")
+        .attachment(EmailParameter.Email.Attachment.builder()
+                .contentId("header-logo")
+                .fileName("logo.png")
+                .contentType("image/png")
+                .payload(new ClassPathResource("/templates/mail/images/logo.png").getInputStream())
+                .build())
+        .build();
+// @formatter:on
+```
+
+Attachments without a `contentId` are added as regular attachments. Each `contentId` must be unique and referenced
+in the `htmlBody` as `cid:contentId`, otherwise validation fails.
+
 ## Usage
 
 ### Sending an Email
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailAttachmentDto.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailAttachmentDto.java
index dbf6f2d..99574ea 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailAttachmentDto.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailAttachmentDto.java
@@ -2,6 +2,7 @@
 
 import it.aboutbits.springboot.emailservice.lib.model.Email;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 @NullMarked
 public record EmailAttachmentDto(
@@ -13,6 +14,9 @@ public record EmailAttachmentDto(
 
         String contentType,
 
+        @Nullable
+        String contentId,
+
         long fileReference
 ) {
 }
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailDto.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailDto.java
index eefe73c..7528009 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailDto.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/EmailDto.java
@@ -26,6 +26,8 @@ public record EmailDto(
         List<String> recipients,
 
         String textBody,
+
+        @Nullable
         String htmlBody,
 
         Set<EmailAttachmentDto> attachments,
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailParameter.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailParameter.java
index 841032d..5a131c4 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailParameter.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailParameter.java
@@ -1,5 +1,7 @@
 package it.aboutbits.springboot.emailservice.lib.application;
 
+import jakarta.validation.Valid;
+import jakarta.validation.constraints.AssertTrue;
 import jakarta.validation.constraints.NotBlank;
 import jakarta.validation.constraints.NotEmpty;
 import lombok.Builder;
@@ -9,13 +11,17 @@
 
 import java.io.InputStream;
 import java.time.OffsetDateTime;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Objects;
 import java.util.Set;
+import java.util.regex.Pattern;
 
 @Builder
 @NullMarked
 public record EmailParameter(
         OffsetDateTime scheduledAt,
+        @Valid
         Email email
 ) {
     @Builder
@@ -28,6 +34,8 @@ public record Email(
             List<String> recipients,
 
             String textBody,
+
+            @Nullable
             String htmlBody,
 
             @NotBlank
@@ -41,15 +49,58 @@ public record Email(
             String replyToName,
 
             @Singular
+            @Valid
             Set<Attachment> attachments
     ) {
+        // the "cid:" scheme is case-insensitive, the contentId itself is not
+        private static final Pattern CID_REFERENCE_PATTERN = Pattern.compile(
+                "cid:([^\\s\"'<>]+)",
+                Pattern.CASE_INSENSITIVE
+        );
+
+        @AssertTrue(message = "each inline attachment contentId must be referenced in the htmlBody as cid:contentId")
+        public boolean isInlineAttachmentsValid() {
+            var contentIds = inlineContentIds();
+            if (contentIds.isEmpty()) {
+                return true;
+            }
+
+            if (htmlBody == null) {
+                return false;
+            }
+
+            var referencedContentIds = new HashSet<String>();
+            var matcher = CID_REFERENCE_PATTERN.matcher(htmlBody);
+            while (matcher.find()) {
+                referencedContentIds.add(matcher.group(1));
+            }
+
+            return referencedContentIds.containsAll(contentIds);
+        }
+
+        @AssertTrue(message = "each inline attachment contentId must be unique")
+        public boolean isInlineAttachmentContentIdsUnique() {
+            var contentIds = inlineContentIds();
+            return contentIds.size() == new HashSet<>(contentIds).size();
+        }
+
+        private List<String> inlineContentIds() {
+            return attachments.stream()
+                    .map(Attachment::contentId)
+                    .filter(Objects::nonNull)
+                    .toList();
+        }
+
         @Builder
         public record Attachment(
                 InputStream payload,
                 @NotBlank
                 String fileName,
                 @NotBlank
-                String contentType
+                String contentType,
+                // if set, the attachment is embedded inline and can be referenced in the htmlBody as "cid:contentId"
+                @Nullable
+                String contentId
         ) {
         }
     }
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailServiceMigrator.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailServiceMigrator.java
index 722e745..66d3b79 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailServiceMigrator.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailServiceMigrator.java
@@ -76,6 +76,8 @@ updated_at           timestamp with time zone default now()           not null,
 
                  alter table email_service_emails add column if not exists cleanup_start_time timestamp with time zone;
 
+                 alter table email_service_email_attachments add column if not exists content_id text;
+
                  update email_service_emails
                     set execution_end_time = sent_at
                   where sent_at is not null
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmail.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmail.java
index 12cc6ce..ab86d16 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmail.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmail.java
@@ -22,6 +22,7 @@
 import org.springframework.transaction.TransactionDefinition;
 import org.springframework.transaction.annotation.Transactional;
 import org.springframework.transaction.support.TransactionTemplate;
+import org.springframework.util.FileCopyUtils;
 import org.springframework.validation.annotation.Validated;
 
 import java.io.IOException;
@@ -183,6 +184,7 @@ private void sendMail(
             String replyToName,
             List<String> recipients,
             String subject,
+            @Nullable
             String htmlBody,
             String plainTextBody,
             Set<EmailAttachment> attachments
@@ -202,16 +204,23 @@ private void sendMail(
             }
         }
 
-        if (!htmlBody.isBlank()) {
+        if (htmlBody != null && !htmlBody.isBlank()) {
             helper.setText(plainTextBody, htmlBody);
         } else {
             helper.setText(plainTextBody);
         }
 
         for (var attachment : attachments) {
-            var payload = attachmentDataSource.getAttachmentPayload(attachment.getFileReference());
-            helper.addAttachment(attachment.getFileName(), new ByteArrayResource(payload.readAllBytes()));
-            payload.close();
+            var resource = new ByteArrayResource(FileCopyUtils.copyToByteArray(
+                    attachmentDataSource.getAttachmentPayload(attachment.getFileReference())
+            ));
+
+            var contentId = attachment.getContentId();
+            if (contentId != null) {
+                helper.addInline(contentId, attachment.getFileName(), resource, attachment.getContentType());
+            } else {
+                helper.addAttachment(attachment.getFileName(), resource, attachment.getContentType());
+            }
         }
 
         mailSender.send(message);
@@ -241,6 +250,7 @@ private Email fromParameter(EmailParameter parameter) throws AttachmentException
             var emailAttachment = new EmailAttachment();
             emailAttachment.setEmail(email);
             emailAttachment.setContentType(attachment.contentType());
+            emailAttachment.setContentId(attachment.contentId());
             emailAttachment.setFileName(attachment.fileName());
             emailAttachment.setFileReference(reference);
 
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailAttachment.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailAttachment.java
index b089bb9..bdc00ad 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailAttachment.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailAttachment.java
@@ -29,5 +29,7 @@ public class EmailAttachment {
 
     private String contentType;
 
+    private String contentId;
+
     private long fileReference;
 }
diff --git a/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailContent.java b/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailContent.java
index dc68116..3ec09f9 100644
--- a/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailContent.java
+++ b/src/main/java/it/aboutbits/springboot/emailservice/lib/model/EmailContent.java
@@ -25,6 +25,8 @@ public record EmailContent(
         List<String> recipients,
 
         String textBody,
+
+        @Nullable
         String htmlBody
 ) {
 }
diff --git a/src/test/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmailTest.java b/src/test/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmailTest.java
index 1c6b9e8..c2d83e0 100644
--- a/src/test/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmailTest.java
+++ b/src/test/java/it/aboutbits/springboot/emailservice/lib/application/ManageEmailTest.java
@@ -5,10 +5,18 @@
 import it.aboutbits.springboot.emailservice.lib.exception.AttachmentException;
 import it.aboutbits.springboot.emailservice.lib.exception.EmailException;
 import it.aboutbits.springboot.emailservice.support.database.WithPostgres;
+import jakarta.mail.Part;
+import jakarta.mail.internet.MimeBodyPart;
 import jakarta.mail.internet.MimeMessage;
+import jakarta.mail.internet.MimeMultipart;
+import jakarta.validation.ConstraintViolationException;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import org.mockito.ArgumentCaptor;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.mail.MailSendException;
@@ -18,6 +26,8 @@
 
 import java.io.ByteArrayInputStream;
 import java.time.OffsetDateTime;
+import java.util.ArrayList;
+import java.util.List;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -153,6 +163,106 @@ void givenRequiredParameterWithAttachedFiles_sendOrFail_shouldSendImmediately()
         verify(javaMailSender, times(1)).send(any(MimeMessage.class));
     }
 
+    @Test
+    void givenInlineAttachment_schedule_shouldPersistContentId() throws EmailException, AttachmentException {
+        when(attachmentDataSource.storeAttachmentPayload(any())).thenReturn(33L);
+
+        var parameter = getValidParameterWithInlineAttachment(
+                "<h1>Html email body</h1><img src=\"cid:header-logo\">",
+                inlineAttachment("header-logo")
+        );
+
+        var result = manageEmail.schedule(parameter);
+
+        assertThat(result.id()).isPositive();
+        assertThat(result.state()).isEqualTo(EmailState.PENDING);
+        assertThat(result.attachments()).hasSize(1);
+        assertThat(result.attachments().iterator().next().contentId()).isEqualTo("header-logo");
+    }
+
+    @Test
+    void givenInlineAndRegularAttachment_sendOrFail_shouldAddInlineAndRegularMimeParts() throws Exception {
+        when(attachmentDataSource.storeAttachmentPayload(any())).thenReturn(33L);
+        when(attachmentDataSource.getAttachmentPayload(anyLong()))
+                .thenAnswer(_ -> new ByteArrayInputStream(new byte[]{1, 2, 3}));
+
+        var parameter = getValidParameterWithInlineAttachment(
+                "<h1>Html email body</h1><img src=\"cid:header-logo\">",
+                inlineAttachment("header-logo"),
+                regularAttachment("x.png")
+        );
+
+        var result = manageEmail.sendOrFail(parameter);
+
+        assertThat(result.state()).isEqualTo(EmailState.SENT);
+
+        var captor = ArgumentCaptor.forClass(MimeMessage.class);
+        verify(javaMailSender).send(captor.capture());
+        var message = captor.getValue();
+        message.saveChanges();
+
+        assertThat(message.getContentType()).startsWith("multipart/mixed");
+
+        var parts = flattenParts(message.getContent());
+
+        var inlinePart = parts.stream()
+                .filter(part -> hasDisposition(part, Part.INLINE))
+                .findFirst()
+                .orElseThrow();
+        assertThat(inlinePart.getContentID()).isEqualTo("<header-logo>");
+        assertThat(inlinePart.getContentType()).startsWith("image/png");
+        assertThat(inlinePart.getFileName()).isEqualTo("logo.png");
+
+        var attachmentPart = parts.stream()
+                .filter(part -> hasDisposition(part, Part.ATTACHMENT))
+                .findFirst()
+                .orElseThrow();
+        assertThat(attachmentPart.getFileName()).isEqualTo("x.png");
+    }
+
+    @Test
+    void givenUppercaseCidReference_schedule_shouldSucceed() throws EmailException, AttachmentException {
+        when(attachmentDataSource.storeAttachmentPayload(any())).thenReturn(33L);
+
+        var parameter = getValidParameterWithInlineAttachment(
+                "<h1>Html email body</h1><img src=\"CID:header-logo\">",
+                inlineAttachment("header-logo")
+        );
+
+        var result = manageEmail.schedule(parameter);
+
+        assertThat(result.state()).isEqualTo(EmailState.PENDING);
+    }
+
+    @ParameterizedTest
+    @CsvSource({
+            "'', header-logo",
+            ", header-logo",
+            "<h1>Html email body</h1>, header-logo",
+            "<img src=cid:header-logo-big>, header-logo",
+            "<h1>Html email body</h1>, ' '"
+    })
+    void givenInvalidInlineAttachment_schedule_shouldFail(@Nullable String htmlBody, String contentId) {
+        var parameter = getValidParameterWithInlineAttachment(htmlBody, inlineAttachment(contentId));
+
+        assertThatExceptionOfType(ConstraintViolationException.class).isThrownBy(
+                () -> manageEmail.schedule(parameter)
+        );
+    }
+
+    @Test
+    void givenDuplicateContentIds_schedule_shouldFail() {
+        var parameter = getValidParameterWithInlineAttachment(
+                "<h1>Html email body</h1><img src=\"cid:header-logo\">",
+                inlineAttachment("header-logo"),
+                inlineAttachment("header-logo", "other.png")
+        );
+
+        assertThatExceptionOfType(ConstraintViolationException.class).isThrownBy(
+                () -> manageEmail.schedule(parameter)
+        );
+    }
+
     @Test
     void givenAttachmentError_sendOrFail_shouldFail() throws EmailException, AttachmentException {
         when(attachmentDataSource.storeAttachmentPayload(any())).thenThrow(new AttachmentException());
@@ -182,16 +292,16 @@ private static EmailParameter getValidParameterWithoutAttachment() {
         return EmailParameter.builder()
                 .scheduledAt(OffsetDateTime.now())
                 .email(EmailParameter.Email.builder()
-                        .subject("Example email subject")
-                        .textBody("Email body")
-                        .htmlBody("<h1>Html email body</h1>")
-                        .recipient("person1@example.com")
-                        .recipient("person2@example.com")
-                        .fromAddress("somebody@aboutbits.it")
-                        .fromName("somebody")
-                        .replyToAddress("somebodyElse@aboutbits.it")
-                        .replyToName("somebodyElse")
-                        .build()
+                               .subject("Example email subject")
+                               .textBody("Email body")
+                               .htmlBody("<h1>Html email body</h1>")
+                               .recipient("person1@example.com")
+                               .recipient("person2@example.com")
+                               .fromAddress("somebody@aboutbits.it")
+                               .fromName("somebody")
+                               .replyToAddress("somebodyElse@aboutbits.it")
+                               .replyToName("somebodyElse")
+                               .build()
                 ).build();
     }
 
@@ -199,23 +309,92 @@ private static EmailParameter getValidParameterWithAttachment() {
         return EmailParameter.builder()
                 .scheduledAt(OffsetDateTime.now())
                 .email(EmailParameter.Email.builder()
-                        .subject("Example email subject")
-                        .textBody("Email body")
-                        .htmlBody("<h1>Html email body</h1>")
-                        .recipient("person1@example.com")
-                        .recipient("person2@example.com")
-                        .attachment(
-                                EmailParameter.Email.Attachment.builder()
-                                        .contentType("image/png")
-                                        .fileName("x.png")
-                                        .payload(new ByteArrayInputStream(new byte[0]))
-                                        .build()
-                        )
-                        .fromAddress("somebody@aboutbits.it")
-                        .fromName("somebody")
-                        .replyToAddress("somebodyElse@aboutbits.it")
-                        .replyToName("somebodyElse")
-                        .build()
+                               .subject("Example email subject")
+                               .textBody("Email body")
+                               .htmlBody("<h1>Html email body</h1>")
+                               .recipient("person1@example.com")
+                               .recipient("person2@example.com")
+                               .attachment(
+                                       EmailParameter.Email.Attachment.builder()
+                                               .contentType("image/png")
+                                               .fileName("x.png")
+                                               .payload(new ByteArrayInputStream(new byte[0]))
+                                               .build()
+                               )
+                               .fromAddress("somebody@aboutbits.it")
+                               .fromName("somebody")
+                               .replyToAddress("somebodyElse@aboutbits.it")
+                               .replyToName("somebodyElse")
+                               .build()
                 ).build();
     }
+
+    private static EmailParameter getValidParameterWithInlineAttachment(
+            @Nullable String htmlBody,
+            EmailParameter.Email.Attachment... attachments
+    ) {
+        var emailBuilder = EmailParameter.Email.builder()
+                .subject("Example email subject")
+                .textBody("Email body")
+                .recipient("person1@example.com")
+                .fromAddress("somebody@aboutbits.it")
+                .fromName("somebody");
+
+        if (htmlBody != null) {
+            emailBuilder.htmlBody(htmlBody);
+        }
+        for (var attachment : attachments) {
+            emailBuilder.attachment(attachment);
+        }
+
+        return EmailParameter.builder()
+                .scheduledAt(OffsetDateTime.now())
+                .email(emailBuilder.build())
+                .build();
+    }
+
+    private static EmailParameter.Email.Attachment inlineAttachment(String contentId) {
+        return inlineAttachment(contentId, "logo.png");
+    }
+
+    private static EmailParameter.Email.Attachment inlineAttachment(String contentId, String fileName) {
+        return EmailParameter.Email.Attachment.builder()
+                .contentType("image/png")
+                .fileName(fileName)
+                .contentId(contentId)
+                .payload(new ByteArrayInputStream(new byte[]{1, 2, 3}))
+                .build();
+    }
+
+    private static EmailParameter.Email.Attachment regularAttachment(String fileName) {
+        return EmailParameter.Email.Attachment.builder()
+                .contentType("image/png")
+                .fileName(fileName)
+                .payload(new ByteArrayInputStream(new byte[]{1, 2, 3}))
+                .build();
+    }
+
+    private static List<MimeBodyPart> flattenParts(Object content) throws Exception {
+        var parts = new ArrayList<MimeBodyPart>();
+        if (content instanceof MimeMultipart multipart) {
+            for (var i = 0; i < multipart.getCount(); i++) {
+                var part = (MimeBodyPart) multipart.getBodyPart(i);
+                var partContent = part.getContent();
+                if (partContent instanceof MimeMultipart) {
+                    parts.addAll(flattenParts(partContent));
+                } else {
+                    parts.add(part);
+                }
+            }
+        }
+        return parts;
+    }
+
+    private static boolean hasDisposition(MimeBodyPart part, String disposition) {
+        try {
+            return disposition.equalsIgnoreCase(part.getDisposition());
+        } catch (Exception e) {
+            throw new IllegalStateException(e);
+        }
+    }
 }