Late last week somebody on my box was complaining that they could receive email but not send it. The strange thing is that it was happening from two separate machines that he swears that he didn't touch the settings on. Both use AOL dialup.
After trying to track down the problem and checking the machine in person it seems that AOL is blocking SMTP traffic to my server. I can't telnet to port 25, though I can send mail to another mail server (which has identical settings) on the same subnet. It seems that AOL uses a transparent SMTP proxy which is just not making the connection, checking with tcpdump confirms that no attempt is made to actually connect.
Any ideas? He's already called AOL who claim that they aren't blocking the connection.