diff --git a/apps/api/src/services/EmailService.ts b/apps/api/src/services/EmailService.ts
index 99dec73..bc40a38 100644
--- a/apps/api/src/services/EmailService.ts
+++ b/apps/api/src/services/EmailService.ts
@@ -18,6 +18,8 @@ interface Attachment {
   filename: string;
   content: string; // Base64 encoded
   contentType: string;
+  contentId?: string;
+  disposition?: 'attachment' | 'inline';
 }
 
 interface SendEmailParams {
@@ -381,7 +383,13 @@ export class EmailService {
       // Parse attachments from JSON
       const attachments =
         email.attachments && Array.isArray(email.attachments)
-          ? (email.attachments as Array<{filename: string; content: string; contentType: string}>)
+          ? (email.attachments as Array<{
+              filename: string;
+              content: string;
+              contentType: string;
+              contentId?: string;
+              disposition?: 'attachment' | 'inline';
+            }>)
           : undefined;
 
       // Determine tracking based on project settings and email type
diff --git a/apps/api/src/services/SESService.ts b/apps/api/src/services/SESService.ts
index ef05b9a..d96cd73 100644
--- a/apps/api/src/services/SESService.ts
+++ b/apps/api/src/services/SESService.ts
@@ -40,6 +40,8 @@ interface SendRawEmailParams {
         filename: string;
         content: string; // Base64 encoded
         contentType: string;
+        contentId?: string;
+        disposition?: 'attachment' | 'inline';
       }[]
     | null;
   tracking?: boolean;
@@ -101,8 +103,13 @@ export async function sendRawEmail({
   }
 
   // Generate unique boundaries for multipart messages
-  const boundary = `----=_NextPart_${Math.random().toString(36).substring(2)}`;
-  const mixedBoundary = attachments?.length ? `----=_MixedPart_${Math.random().toString(36).substring(2)}` : null;
+  const altBoundary = `----=_AltPart_${Math.random().toString(36).substring(2)}`;
+  const mixedBoundary = attachments?.some(a => (a.disposition ?? 'attachment') === 'attachment')
+    ? `----=_MixedPart_${Math.random().toString(36).substring(2)}`
+    : null;
+  const relatedBoundary = attachments?.some(a => a.disposition === 'inline')
+    ? `----=_RelatedPart_${Math.random().toString(36).substring(2)}`
+    : null;
 
   // Format To header with names if provided
   const toHeader = to
@@ -118,17 +125,21 @@ export async function sendRawEmail({
   // Extract just email addresses for Destinations (SES requirement)
   const destinations = to.map(recipient => (typeof recipient === 'string' ? recipient : recipient.email));
 
+  // Determine root content type
+  let rootContentType = `multipart/alternative; boundary="${altBoundary}"`;
+  if (mixedBoundary) {
+    rootContentType = `multipart/mixed; boundary="${mixedBoundary}"`;
+  } else if (relatedBoundary) {
+    rootContentType = `multipart/related; boundary="${relatedBoundary}"`;
+  }
+
   // Build raw MIME message
-  const rawMessage = `From: ${from.name} <${from.email}>
+  let rawMessage = `From: ${from.name} <${from.email}>
 To: ${toHeader}
 Reply-To: ${reply || from.email}
 Subject: ${content.subject}
 MIME-Version: 1.0
-${
-  mixedBoundary
-    ? `Content-Type: multipart/mixed; boundary="${mixedBoundary}"`
-    : `Content-Type: multipart/alternative; boundary="${boundary}"`
-}
+Content-Type: ${rootContentType}
 ${
   headers
     ? Object.entries(headers)
@@ -138,29 +149,61 @@ ${
 }
 ${unsubscribeHeader}
 
-${mixedBoundary ? `--${mixedBoundary}\n` : ''}${
-    mixedBoundary ? `Content-Type: multipart/alternative; boundary="${boundary}"\n\n` : ''
-  }--${boundary}
+`;
+
+  // building the body
+  if (mixedBoundary) {
+    rawMessage += `--${mixedBoundary}\n`;
+    if (relatedBoundary) {
+      rawMessage += `Content-Type: multipart/related; boundary="${relatedBoundary}"\n\n`;
+      rawMessage += `--${relatedBoundary}\n`;
+    }
+  } else if (relatedBoundary) {
+    rawMessage += `--${relatedBoundary}\n`;
+  }
+
+  // If we are nested, we need to specify that this next part is the alternative container
+  if (mixedBoundary || relatedBoundary) {
+    rawMessage += `Content-Type: multipart/alternative; boundary="${altBoundary}"\n\n`;
+  }
+
+  // The alternative part content (always contains HTML)
+  rawMessage += `--${altBoundary}
 Content-Type: text/html; charset=utf-8
 Content-Transfer-Encoding: 7bit
 
 ${breakLongLines(content.html, 500)}
---${boundary}--
-${
-  attachments && attachments.length > 0
-    ? '\n' +
-      attachments
-        .map(
-          attachment => `--${mixedBoundary}
+--${altBoundary}--
+`;
+
+  // Add inline attachments to the related container
+  if (relatedBoundary) {
+    const inlineAttachments = attachments?.filter(a => a.disposition === 'inline') ?? [];
+    for (const attachment of inlineAttachments) {
+      rawMessage += `\n--${relatedBoundary}
+Content-Type: ${attachment.contentType}
+Content-Transfer-Encoding: base64
+Content-ID: <${attachment.contentId || attachment.filename}>
+Content-Disposition: inline; filename="${attachment.filename}"
+
+${breakLongLines(attachment.content, 76, true)}`;
+    }
+    rawMessage += `\n--${relatedBoundary}--`;
+  }
+
+  // Add regular attachments to the mixed container
+  if (mixedBoundary) {
+    const regularAttachments = attachments?.filter(a => (a.disposition ?? 'attachment') === 'attachment') ?? [];
+    for (const attachment of regularAttachments) {
+      rawMessage += `\n--${mixedBoundary}
 Content-Type: ${attachment.contentType}
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename="${attachment.filename}"
 
-${breakLongLines(attachment.content, 76, true)}`,
-        )
-        .join('\n')
-    : ''
-}${mixedBoundary ? `\n--${mixedBoundary}--` : ''}`;
+${breakLongLines(attachment.content, 76, true)}`;
+    }
+    rawMessage += `\n--${mixedBoundary}--`;
+  }
 
   // Determine which configuration set to use
   // Only use NO_TRACKING if tracking toggle is enabled AND tracking is disabled
diff --git a/apps/api/src/services/__tests__/EmailService.test.ts b/apps/api/src/services/__tests__/EmailService.test.ts
index 0d6e141..4db3cc5 100644
--- a/apps/api/src/services/__tests__/EmailService.test.ts
+++ b/apps/api/src/services/__tests__/EmailService.test.ts
@@ -1,11 +1,32 @@
-import {beforeEach, describe, expect, it, vi} from 'vitest';
+import {beforeEach, describe, expect, it, vi, type Mock} from 'vitest';
 import {EmailSourceType, EmailStatus} from '@plunk/db';
 import {ActionSchemas} from '@plunk/shared';
 import {EmailService} from '../EmailService';
 import {sendRawEmail} from '../SESService';
 import {factories, getPrismaClient} from '../../../../../test/helpers';
 
-// Mock SES service
+// Mock AWS SDK globally (used by real SESService calls in MIME tests)
+vi.mock('@aws-sdk/client-ses', () => {
+  const SESMock = vi.fn();
+  SESMock.prototype.sendRawEmail = vi.fn().mockResolvedValue({MessageId: 'test-message-id'});
+  return {SES: SESMock};
+});
+
+// Mock constants to provide AWS credentials for SESService, preserving other exports
+vi.mock('../../app/constants.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../../app/constants.js')>();
+  return {
+    ...actual,
+    AWS_SES_ACCESS_KEY_ID: 'test-key-id',
+    AWS_SES_REGION: 'us-east-1',
+    AWS_SES_SECRET_ACCESS_KEY: 'test-secret',
+    SES_CONFIGURATION_SET: 'test-config-set',
+    SES_CONFIGURATION_SET_NO_TRACKING: 'test-no-tracking-set',
+    TRACKING_TOGGLE_ENABLED: true,
+  };
+});
+
+// Mock SES service (default behavior for most tests)
 vi.mock('../SESService', () => ({
   sendRawEmail: vi.fn(),
 }));
@@ -801,5 +822,204 @@ describe('EmailService', () => {
         expect(result.success).toBe(true);
       }
     });
+
+    it('should accept inline attachment with contentId', () => {
+      const result = ActionSchemas.send.safeParse({
+        to: 'test@example.com',
+        from: 'test@example.com',
+        subject: 'Inline Image',
+        body: '<img src="cid:logo" />',
+        attachments: [
+          {
+            filename: 'logo.png',
+            content: Buffer.from('image').toString('base64'),
+            contentType: 'image/png',
+            contentId: 'logo',
+            disposition: 'inline',
+          },
+        ],
+      });
+
+      expect(result.success).toBe(true);
+      if (result.success) {
+        const attachment = result.data.attachments![0];
+        expect(attachment.contentId).toBe('logo');
+        expect(attachment.disposition).toBe('inline');
+      }
+    });
+
+    it('should reject contentId exceeding 255 chars', () => {
+      const result = ActionSchemas.send.safeParse({
+        to: 'test@example.com',
+        subject: 'Test',
+        body: 'Test',
+        attachments: [
+          {
+            filename: 'image.png',
+            content: Buffer.from('content').toString('base64'),
+            contentType: 'image/png',
+            contentId: 'a'.repeat(256),
+            disposition: 'inline',
+          },
+        ],
+      });
+
+      expect(result.success).toBe(false);
+    });
+
+    it('should reject invalid disposition', () => {
+      const result = ActionSchemas.send.safeParse({
+        to: 'test@example.com',
+        subject: 'Test',
+        body: 'Test',
+        attachments: [
+          {
+            filename: 'image.png',
+            content: Buffer.from('content').toString('base64'),
+            contentType: 'image/png',
+            disposition: 'invalid-disposition',
+          },
+        ],
+      });
+
+      expect(result.success).toBe(false);
+    });
+
+  });
+});
+
+// ========================================
+// SES MIME BOUNDARY STRUCTURE
+// ========================================
+// These tests verify the raw MIME assembly logic inside sendRawEmail.
+// They need the REAL sendRawEmail (not the mock above), so we mock
+// at the AWS SDK level instead.
+
+describe('SES MIME Boundary Structure', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('should correctly structure MIME boundaries for mixed content (attachments)', async () => {
+    const {sendRawEmail: realSendRawEmail, ses} = await vi.importActual<typeof import('../SESService')>('../SESService');
+
+    const params = {
+      from: {name: 'Sender', email: 'sender@example.com'},
+      to: ['recipient@example.com'],
+      content: {subject: 'Test Subject', html: '<p>Hello world</p>'},
+      attachments: [
+        {
+          filename: 'test.txt',
+          content: 'SGVsbG8=',
+          contentType: 'text/plain',
+          disposition: 'attachment' as const,
+        },
+      ],
+    };
+
+    await realSendRawEmail(params);
+
+    expect(ses.sendRawEmail).toHaveBeenCalled();
+    const callArgs = (ses.sendRawEmail as Mock).mock.calls[0][0];
+    const rawMessage = new TextDecoder().decode(callArgs.RawMessage.Data);
+
+    // Verify boundary hierarchy: Mixed -> Alternative
+    expect(rawMessage).toMatch(/^From:.*Content-Type: multipart\/mixed; boundary="([^"]+)"/s);
+    expect(rawMessage).toMatch(/Content-Type: multipart\/alternative; boundary="([^"]+)"/);
+
+    const mixedBoundaryMatch = rawMessage.match(/boundary="([^"]+)"/);
+    const mixedBoundary = mixedBoundaryMatch ? mixedBoundaryMatch[1] : '';
+
+    expect(rawMessage).toContain(`--${mixedBoundary}\nContent-Type: multipart/alternative`);
+    expect(rawMessage).toContain(`--${mixedBoundary}--`);
+  });
+
+  it('should correctly structure MIME boundaries for related content (inline images)', async () => {
+    const {sendRawEmail: realSendRawEmail, ses} = await vi.importActual<typeof import('../SESService')>('../SESService');
+
+    const params = {
+      from: {name: 'Sender', email: 'sender@example.com'},
+      to: ['recipient@example.com'],
+      content: {subject: 'Test Subject', html: '<p>Hello world <img src="cid:image1"></p>'},
+      attachments: [
+        {
+          filename: 'image.png',
+          content:
+            'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=',
+          contentType: 'image/png',
+          contentId: 'image1',
+          disposition: 'inline' as const,
+        },
+      ],
+    };
+
+    await realSendRawEmail(params);
+
+    const callArgs = (ses.sendRawEmail as Mock).mock.calls[0][0];
+    const rawMessage = new TextDecoder().decode(callArgs.RawMessage.Data);
+
+    // Verify boundary hierarchy: Related -> Alternative
+    expect(rawMessage).toMatch(/^From:.*Content-Type: multipart\/related; boundary="([^"]+)"/s);
+
+    const relatedBoundaryMatch = rawMessage.match(/boundary="([^"]+)"/);
+    const relatedBoundary = relatedBoundaryMatch ? relatedBoundaryMatch[1] : '';
+
+    expect(rawMessage).toContain(`--${relatedBoundary}\nContent-Type: multipart/alternative`);
+    expect(rawMessage).toContain(`Content-Disposition: inline; filename="image.png"`);
+    expect(rawMessage).toContain(`--${relatedBoundary}--`);
+  });
+
+  it('should correctly nest mixed > related > alternative boundaries', async () => {
+    const {sendRawEmail: realSendRawEmail, ses} = await vi.importActual<typeof import('../SESService')>('../SESService');
+
+    const params = {
+      from: {name: 'Sender', email: 'sender@example.com'},
+      to: ['recipient@example.com'],
+      content: {subject: 'Test Subject', html: '<p>Hello world <img src="cid:image1"></p>'},
+      attachments: [
+        {
+          filename: 'test.txt',
+          content: 'SGVsbG8=',
+          contentType: 'text/plain',
+          disposition: 'attachment' as const,
+        },
+        {
+          filename: 'image.png',
+          content:
+            'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=',
+          contentType: 'image/png',
+          contentId: 'image1',
+          disposition: 'inline' as const,
+        },
+      ],
+    };
+
+    await realSendRawEmail(params);
+
+    const callArgs = (ses.sendRawEmail as Mock).mock.calls[0][0];
+    const rawMessage = new TextDecoder().decode(callArgs.RawMessage.Data);
+
+    // Root should be mixed
+    expect(rawMessage).toMatch(/^From:.*Content-Type: multipart\/mixed; boundary="([^"]+)"/s);
+
+    const mixedMatch = rawMessage.match(/Content-Type: multipart\/mixed; boundary="([^"]+)"/);
+    const mixedBoundary = mixedMatch ? mixedMatch[1] : 'NOT_FOUND_MIXED';
+
+    // Within mixed, we should find related
+    expect(rawMessage).toContain(`--${mixedBoundary}\nContent-Type: multipart/related`);
+
+    const relatedMatch = rawMessage.match(/Content-Type: multipart\/related; boundary="([^"]+)"/);
+    const relatedBoundary = relatedMatch ? relatedMatch[1] : 'NOT_FOUND_RELATED';
+
+    // Within related, we should find alternative
+    expect(rawMessage).toContain(`--${relatedBoundary}\nContent-Type: multipart/alternative`);
+
+    const altMatch = rawMessage.match(/Content-Type: multipart\/alternative; boundary="([^"]+)"/);
+    const altBoundary = altMatch ? altMatch[1] : 'NOT_FOUND_ALT';
+
+    // Verify all closing boundaries exist
+    expect(rawMessage).toContain(`--${altBoundary}--`);
+    expect(rawMessage).toContain(`--${relatedBoundary}--`);
+    expect(rawMessage).toContain(`--${mixedBoundary}--`);
   });
 });
diff --git a/packages/shared/src/schemas/index.ts b/packages/shared/src/schemas/index.ts
index 7fa35b0..9eadd33 100644
--- a/packages/shared/src/schemas/index.ts
+++ b/packages/shared/src/schemas/index.ts
@@ -407,7 +407,18 @@ export const ActionSchemas = {
             filename: z.string().min(1).max(255),
             content: z.string().min(1), // Base64 encoded file content
             contentType: z.string().min(1).max(255),
-          }),
+            contentId: z
+              .string()
+              .min(1)
+              .max(255)
+              .regex(/^[^<>\r\n]+$/, 'Content ID cannot contain <, >, \\r, or \\n')
+              .optional(),
+            disposition: z.enum(['attachment', 'inline']).default('attachment'),
+          })
+            .refine(data => data.disposition !== 'inline' || !!data.contentId, {
+              message: 'Content ID is required when disposition is inline',
+              path: ['contentId'],
+            }),
         )
         .max(10) // Maximum 10 attachments per email
         .optional(),