Why am I getting an "SMTP send failed to [Response: Unable to connect; Code: 11001]" error?

Issue:

Some outbound emails failed to send because the Email Gateway Security couldn't connect to the recipient's mail server.

SMTP send failed to [Response: Unable to connect; Code: 11001]
smtp://domain.mail.protection.outlook.com:25

This error indicates that the sending server was unable to connect to the recipient's mail server. The most likely causes include:

• DNS resolution failure for the domain domain.mail.protection.outlook.com

• Firewall or network restrictions blocking outbound SMTP traffic on port 25

• Temporary service interruption on Microsoft’s mail protection infrastructure (Exchange Online)

The system automatically retried sending the messages, but all attempts failed due to the same connectivity error.

Resolution:

Check DNS Resolution, Review Firewall Rules, Check Microsoft Service Status, and Retry Email Delivery.

Steps:

1. Test Port 25 Connectivity (Use telnet to test if your server can connect to the destination on port 25)

   telnet domain.mail.protection.outlook.com 25

   • A successful connection will return something like:

     • 220 domain.mail.protection.outlook.com Microsoft ESMTP MAIL Service ready at Wed, 30 Jul 2025 05:35:57 +0000 [08DDCC5163718589]

   • If it fails to connect or times out, port 25 may be blocked or filtered.

2. Check DNS Resolution

   Make sure the domain can be resolved from your sending server:

   nslookup domain.mail.protection.outlook.com

3. Review Firewall Rules

   Ensure that outbound traffic on TCP port 25 is allowed. Some hosting providers (especially cloud platforms) may block this port by default.

4. Check Microsoft Service Status

   Visit Microsoft 365 Status to see if there's a known issue affecting mail delivery.

5. Retry Email Delivery

   Once DNS and connectivity issues are resolved, resending the affected messages should succeed.