Bounceback received on outbound relay. SMTP error message: "delivery temporarily suspended: connect to [XXX.XXX.XXX.XXX]:25: Connection timed out.
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.
Resolution - How to identify the point of failure
-Review email logs
To access email logs:
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
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.