diff --git a/src/queue-manager.js b/src/queue-manager.js
index 6fdf0ee..4b166e3 100644
--- a/src/queue-manager.js
+++ b/src/queue-manager.js
@@ -40,13 +40,39 @@ export class QueueManager {
       {
         store: new SQLiteStore({
           path: this.dbPath,
-          // Store email
-          serialize: (data) => JSON.stringify(data),
-          deserialize: (text) => JSON.parse(text),
+          // Buffer handling
+          serialize: (data) => {
+            // Convert attachment buffers to base64
+            const serializable = { ...data };
+            if (serializable.attachments) {
+              serializable.attachments = serializable.attachments.map((att) => ({
+                ...att,
+                content: att.content && Buffer.isBuffer(att.content)
+                  ? att.content.toString('base64')
+                  : att.content,
+                _isBuffer: Buffer.isBuffer(att.content),
+              }));
+            }
+            return JSON.stringify(serializable);
+          },
+          deserialize: (text) => {
+            const data = JSON.parse(text);
+            // Convert base64 back to Buffers
+            if (data.attachments) {
+              data.attachments = data.attachments.map((att) => ({
+                ...att,
+                content: att._isBuffer && typeof att.content === 'string'
+                  ? Buffer.from(att.content, 'base64')
+                  : att.content,
+              }));
+              // Clean up
+              data.attachments.forEach((att) => delete att._isBuffer);
+            }
+            return data;
+          },
         }),
-        // Retry
+        // Retry configuration
         maxRetries: queueConfig.maxRetries,
-        retryDelay: queueConfig.retryDelay,
         concurrent: 1,
         // Retry exponential backoff
         afterProcessDelay: 100,
diff --git a/src/smtp-client.js b/src/smtp-client.js
index e5a7d94..77e0ba8 100644
--- a/src/smtp-client.js
+++ b/src/smtp-client.js
@@ -50,6 +50,24 @@ export class SMTPClient {
     const transport = this.getTransport(provider);
 
     try {
+      // Validate and sanitize attachments
+      const sanitizedAttachments = (emailData.attachments || []).map((att) => {
+        // Ensure content is a Buffer or string
+        if (att.content && typeof att.content === 'object' && !Buffer.isBuffer(att.content)) {
+          this.logger.warn('Invalid attachment content type, skipping', {
+            filename: att.filename,
+            type: typeof att.content,
+          });
+          return null;
+        }
+        return {
+          filename: att.filename || 'attachment',
+          content: att.content,
+          contentType: att.contentType || 'application/octet-stream',
+          encoding: att.encoding || 'base64',
+        };
+      }).filter(Boolean); // Remove null entries
+
       // Prepare email
       const mailOptions = {
         from: provider.from,
@@ -58,7 +76,7 @@ export class SMTPClient {
         text: emailData.text,
         html: emailData.html,
         headers: emailData.headers || {},
-        attachments: emailData.attachments || [],
+        attachments: sanitizedAttachments,
 
         // Reply-to original sender
         replyTo: emailData.envelope.from,
diff --git a/src/smtp-server.js b/src/smtp-server.js
index a63e71e..a2b3472 100644
--- a/src/smtp-server.js
+++ b/src/smtp-server.js
@@ -119,7 +119,8 @@ export class IncomingSMTPServer {
           content: att.content,
           contentType: att.contentType,
           contentDisposition: att.contentDisposition,
-          size: att.size,
+          contentId: att.contentId,
+          encoding: 'base64',
         })) || [],
         messageId: parsed.messageId,
         date: parsed.date,
@@ -135,6 +136,20 @@ export class IncomingSMTPServer {
         throw new Error('Missing recipient address');
       }
 
+      // Check for email loops (sender sending to themselves)
+      const loopRecipients = emailData.envelope.to.filter(
+        (recipient) => recipient.toLowerCase() === emailData.envelope.from.toLowerCase()
+      );
+
+      if (loopRecipients.length > 0) {
+        this.logger.warn('Email loop detected - dropping email', {
+          from: emailData.envelope.from,
+          to: emailData.envelope.to,
+          loopRecipients,
+        });
+        throw new Error('Email loop detected: sender cannot send to themselves');
+      }
+
       this.logger.debug('Email parsed successfully', {
         from: emailData.envelope.from,
         to: emailData.envelope.to,