PMDF System Manager's Guide

Timeouts on incoming SMTP connections are most often related to system resources and the allocation thereof.

Check how many simultaneous incoming SMTP connections you allow. This is controlled by the MAX_PROCS and MAX_CONNECTIONS Dispatcher settings for the SMTP service; the number of simultaneous connections allowed is MAX_PROCS*MAX_CONNECTIONS. If you can afford the system resources, consider raising this number if it is too low for your usage.

Try putting the slave_debug keyword on the channels handling incoming SMTP over TCP/IP mail, usually tcp_local. Then take a look at the resulting tcp_local_slave.log-uniqueid files, and try to spot any particular characteristics of the messages that time out. For instance, if incoming messages with large numbers of recipients are timing out, consider using the expandlimit keyword on the channel.

Of course, if your system is extremely overloaded and overextended, time outs will be difficult to avoid entirely.