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Proofpoint, Inc.

Bounceback received on outbound relay

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Situation Admin receives report from customer about bounced emails (Outbound direction) with SMTP error message: "delivery temporarily suspended: connect to [XXX.XXX.XXX.XXX]:25: Connection timed out.
Solution Use some tools or commands to make sure that our servers are connecting with recipient server such as telnet, traceroute, etc


Most importantly

The bounceback is required to investigate the logs of the destination SMTP server. Problems of this nature are usually a poor quality SMTP server, a broken firewall or router in place between our servers and the destination server (dropping ICMP packets, for example), or improper MTU configuration on the destination server/network. You can view this information via the outbound logs or via the bounceback email received.

Review email Logs

To access email logs:

  1. Log in to the Proofpoint Essentials user interface
  2. Type the customer name into the search bar (top header).
  3. Click Logs.
  4. Select user (or search for user and select).
  5. Click Logs to the right of the user name.
  6. Change Type to Outbound.
  7. Change Status to Any.
  8. Click Search.
  9. Click Detail button next to the message.

Best Practices for troubleshooting

  • Always check the Delivery: section in the Email Detail window 
  • You can find valuable information to troubleshoot or have an idea what is going on.
Status: Deferred
Summary: This message could not be delivered yet, but will be retried periodically until either success or expiry
Timestamp: 13:43:27 PST, Monday 16 December 2019
Last delivery response: delivery temporarily suspended: connect to abc-defg-jkl.mns.mail.xyz[111.111.1.1]:25: Connection timed out
  • In this box you can see that Proofpoint is trying to hand off an email to recipient's server with IP: 111.111.1.1 through port 25, but the RECIPIENT server is timing out.

Useful Commands

Telnet Connection

One of the first steps to identify the issue would be to perform a telnet connection to the recipient SMTP server on port 25 and confirm if a successful connection to the mail server can be made. telnet [Receiving SMTP Server] 25 

Steps to test connection through Telnet: Telnet Test

Packet Capture 

Using a packet capture tool like Wireshark can help identify where the connection is timing out and may provide more information for the reason why.

Traceroute / TCPTraceroute

Using traceroute or tcptraceroute will display each hop as the packet tries to reach the destination SMTP server. Traceroute relys on ICMP or UDP protocols, although firewalls and routers usually block the ICMP protocol which is why it may be more beneficial to use tcptraceroute which uses the TCP protocol.