While the user is having apparent connection issues, that seem to only be on their end, have them do as I suggested in my previous post:
have them 1: ping the mail server both by IP and name and see if they have a basic connection. 2 - perform an NSLOOKUP up on the mail server to see if they have a DNS problem where they can't resolve the server, 3 - try to telnet into the server on port 25 and see if they get a response at all. Have the connect and possibly enter some HELO information and then exit. This should create an error entry in your mail logs. You should verify. This will help rule out a network issue in which case you can focus on the application and its settings.
In order to figure out what is happening, you will need to get some diagnostic information. The timeout could be caused by either a network connectivity issue, which would explain the intermittent nature. Such an issue could be DNS related and I would have them perform the NSLOOKUP a couple of times. It could be caused by a protocol or port mismatch in their settings. It could even be that they are being blocked by a proxy or a lousy ISP policy somewhere.
Consequently, one of the first things to determine is while they are experiencing problems is if they are able to connect to the server at all or not. If not, you need to find out where the offending device is. Trying a Tracerout and trying to Telnet into the mail server will help in this regard.