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Originally Posted by GandalfTheWhite
It gets one third of the way through and then just hangs. After a few minutes (depending on how long they've set the wait period in their mail app), they get a "Server timed out" error message...
It's nothing on the ISP's end as we'd be getting flooded with calls about not being able to send out by thousands of angry internet users.
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Could it possibly be their TCP/IP settings?
One guy I know had an standard 10/100 ethernet card and, instead of letting it default, had set it to 100 (for his home LAN) -- but then when he tried to hook up to an outside ISP, he had troubles. As I recall, nothing worked, so this may not be it.
One thing I have seen that does have this symptom is MTU setting, fragmentation, and message size. "Large" messages would not get through because the TCP/IP packets were being fragmented and the router would not allow fragmented packets through. If the MTU is tiny, the packets may be fragmenting and somewhere they are having trouble. One way to test this is to see what your MTU is, and then use PING with a packet size greater than the MTU to see if it connects.
Failing that, you will need to run sendmail (or whatever) in debug to watch what is going on.
If attachments fail, I assume that a large email message (without attachments -- just lots of text will also fail).