Had a client with a blocked SMTP server this week.   After taking a look at a few bounce messages it became evident that the connection had just been terminated by the remote host.

Verifying the problem is possible, using telnet to the remote host on port 25 from your local mail server:

telnet <remote mail server> 25

If you get disconnected immediately – you’re probably getting blocked as a spam sender.   Here’s a useful site to check the status of your mail server with a variety of filters:

http://www.mxtoolbox.com/blacklists.aspx

After clearing an erroneous block, mail started flowing again (albeit after a few hours).

I’ll be keeping an eye on the above site for any further warnings on this mail server….