users we have. I don't know much about how email is delivered, how long it takes, etc. But can all this spam really be hurting our net connection?
Not "can", 'is", on your low bandwidth link.
Any MX policy that accepts all DATA commands and then deletes/rejects is wasting significant bandwidth vs legit SMTP bandwidth, since spam is now well over 60% of all inbound, and not infrequently 90+%. The solution/alternative is to reject as much as possible before the DATA command.
but file transfers along can easily congest a 256 Kb/s link.
And note that when a tcp/ip link is congested (and only one direction needs to be congested, since the total link is slowed to the speed of the slower direction), lots of packets timeout, are lost, both UDP (DNS) and TCP, causing more traffic as UDP and TCP, and the apps, are re-tried, re-tried, re-tried, in a deadly spiral.
google for "win mtr" and run it between your Imail box to the IP of the first hop on the other side of your wan link. MTR, "matt's trace route" ported from Unix, will show you stats about the delays per hop, min/max/avg, and lost packets. Note specifically the differences MTR's numbers with and w/o a file transfer in progress.
Len
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