We have one or two specific local senders (notification servers) that are authorized in Xeams as relays on port 25. Other than that, port 25 is blocked as this is configured as a closed relay in Xeams SMTP config. More and more often, in fact almost continuously today, these listed relays fail on port 25 with "500 server is too busy". In the past this maybe happened once every few days, and we rebooted Xeams to fix it. Today it happens almost immediately after we reboot xeams. We can send one or two messages via port 25, and then a packet capture shows we get the "500 server too busy. Try again later" SMTP error returned. We are not running AV, as that is done by a front-end service (MXGuardDog). Oddly, mail from the spam filtering service arrives at port 25 with no problem!

We've looked at all the logs, run the Xeams SMTP diagnostics, all show no problem.

Mel,

Xeams displays the "Server too busy" message if it detects unusual/malicious activity from an IP address, and now that IP is blocked from sending messages. Check SMTPConversation.log to see exactly what you received from that IP.

Refer to https://www.xeams.com/friendlyip.htm and put the two IP addresses where emails are sent from to Xeams. Once you put these in the Friendly IP, Xeams won't block them.