Hello,
this time limit can be set in the GUI under Network -> Listeners -> Edit Global Settings. The parameter you're looking for is most likely the 'Total Time Limit for All Inbound Connections' which is set to 15 minutes by default. However, the posted mail flow is most likely based on a network bandwidth issue where the sending mail server is not able to transmit the data in the session fast enough - or - the message is extremely large in size and cannot be transmitted within 15 minutes.
Even when raising the connection time limit to a higher value seems to be a good idea at the first glance, it is not. Please consider that overly long pending connections will decrease the available pool of 'Maximum Concurrent Connections' configured and you may end up with a decreased performance when increasing the connection time limit without prior analysis why the sending mail server requires too much time to deliver the message in. When the number of concurrent connections is reached, no additional incoming connection will be accepted by the appliance.
In case your 'Timeout for Unsuccessful Inbound Connections' is also configured to 15 minutes (it is 5 minutes by default), then I'd suggest to investigate if Path MTU Discovery (RFC1191) is blocked between the sender and your appliance, as this might also be a valid reason why the sender runs into a timeout as the ICMP response is blocked.
Hope this helps.
Regards,
Martin
