diff --git a/apps/api/src/services/__tests__/SESService.test.ts b/apps/api/src/services/__tests__/SESService.test.ts
new file mode 100644
index 0000000..014e7ad
--- /dev/null
+++ b/apps/api/src/services/__tests__/SESService.test.ts
@@ -0,0 +1,174 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { sendRawEmail } from '../SESService';
+
+// Mock with both paths to catch resolution variations
+vi.mock('../../app/constants.js', () => ({
+  AWS_SES_ACCESS_KEY_ID: 'test-key-id',
+  AWS_SES_REGION: 'us-east-1',
+  AWS_SES_SECRET_ACCESS_KEY: 'test-secret',
+  DASHBOARD_URI: 'http://localhost:3000',
+  SES_CONFIGURATION_SET: 'test-config-set',
+  SES_CONFIGURATION_SET_NO_TRACKING: 'test-no-tracking-set',
+  TRACKING_TOGGLE_ENABLED: true,
+}));
+
+
+
+// Mock the SES client
+vi.mock('@aws-sdk/client-ses', () => {
+  const SESMock = vi.fn();
+  SESMock.prototype.sendRawEmail = vi.fn().mockResolvedValue({ MessageId: 'test-message-id' });
+  return { SES: SESMock };
+});
+
+describe('SESService', () => {
+  let sesInstance: any;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('should correctly structure MIME boundaries for mixed content (attachments)', async () => {
+    // Import the exported instance to verify calls
+    const { ses } = await import('../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=', // Hello
+          contentType: 'text/plain',
+          disposition: 'attachment' as const,
+        },
+      ],
+    };
+
+    await sendRawEmail(params);
+
+    expect(ses.sendRawEmail).toHaveBeenCalled();
+    const callArgs = (ses.sendRawEmail as any).mock.calls[0][0];
+    const rawMessage = new TextDecoder().decode(callArgs.RawMessage.Data);
+
+    // Verify boundary hierarchy: Mixed -> Alternative
+    // Explicitly check that "multipart/mixed" is the root content type (first occurrence)
+    expect(rawMessage).toMatch(/^From:.*Content-Type: multipart\/mixed; boundary="([^"]+)"/s);
+    
+    // Check for presence of alternative boundary nested inside
+    expect(rawMessage).toMatch(/Content-Type: multipart\/alternative; boundary="([^"]+)"/);
+    
+    // Verify distinct boundaries
+    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 { ses } = await import('../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 sendRawEmail(params);
+
+    const callArgs = (ses.sendRawEmail as any).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] : '';
+    
+    // Check that related part contains alternative part
+    expect(rawMessage).toContain(`--${relatedBoundary}\nContent-Type: multipart/alternative`);
+    // And also contains the inline attachment
+    expect(rawMessage).toContain(`Content-Disposition: inline; filename="image.png"`);
+    expect(rawMessage).toContain(`--${relatedBoundary}--`);
+  });
+
+  it('should correctly nest mixed > related > alternative boundaries', async () => {
+    const { ses } = await import('../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 sendRawEmail(params);
+
+    const callArgs = (ses.sendRawEmail as any).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);
+
+    // Find the mixed boundary
+    const mixedMatch = rawMessage.match(/Content-Type: multipart\/mixed; boundary="([^"]+)"/);
+    const mixedBoundary = mixedMatch ? mixedMatch[1] : 'NOT_FOUND_MIXED';
+    
+    // Within mixed, we should find related
+    // The code structure:
+    // --mixed
+    // Content-Type: multipart/related; boundary="related"
+    // --related
+    // Content-Type: multipart/alternative; boundary="alt"
+    
+    // Check hierarchy via regex matching structure
+    // Check that related is defined inside mixed
+    expect(rawMessage).toContain(`--${mixedBoundary}\nContent-Type: multipart/related`);
+    
+    // Find related boundary
+    const relatedMatch = rawMessage.match(/Content-Type: multipart\/related; boundary="([^"]+)"/);
+    const relatedBoundary = relatedMatch ? relatedMatch[1] : 'NOT_FOUND_RELATED';
+    
+    // Check that alternative is defined inside related
+    expect(rawMessage).toContain(`--${relatedBoundary}\nContent-Type: multipart/alternative`);
+    
+    const altMatch = rawMessage.match(/Content-Type: multipart\/alternative; boundary="([^"]+)"/);
+    const altBoundary = altMatch ? altMatch[1] : 'NOT_FOUND_ALT';
+    
+    // Verify closing boundaries existence and order implies nesting
+    // The end of string should look like:
+    // --related--
+    // --mixed
+    // ...attachment...
+    // --mixed--
+    
+    expect(rawMessage).toContain(`--${altBoundary}--`);
+    expect(rawMessage).toContain(`--${relatedBoundary}--`);
+    expect(rawMessage).toContain(`--${mixedBoundary}--`);
+  });
+});
diff --git a/packages/shared/src/schemas/index.ts b/packages/shared/src/schemas/index.ts
index b469914..baa0bd3 100644
--- a/packages/shared/src/schemas/index.ts
+++ b/packages/shared/src/schemas/index.ts
@@ -380,9 +380,18 @@ export const ActionSchemas = {
             filename: z.string().min(1).max(255),
             content: z.string().min(1), // Base64 encoded file content
             contentType: z.string().min(1).max(255),
-            contentId: z.string().min(1).max(255).optional(),
+            contentId: z
+              .string()
+              .min(1)
+              .max(255)
+              .regex(/^[^<>\r\n]+$/, '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(),