The guy wasn't too pleased his email wouldn't arrive. He'd tried several times, so one of the support staff asked him to play our usual trick of emailing us and CC a gmail address so that we have a time fix, and a server name.

Only this time the gmail didn't arrive either- and the guy was clearly getting irate, so I got involved.

After an hour or two, the penny dropped. the outward facing email server at my end (exim - a truly wonderful piece of code) was checking the incoming HELO for syntax. His HELO had an underscore in. So the fix was delightfully simple:


helo_allow_chars = "_"

in the configure file and

killall -HUP exim

at the command prompt.

About 5 mins later, his message was accepted.
But why do I have to break my server from spec? Well, he's a potential customer, with an exchange server and MS trained guys who don't know anything much about the RFCs for hostnames not containing underscore characters

I did have what I think is an effusive email from him. I suspect he's not too chuffed with his techies.

http://en.wikipedia.org/wiki/Anti-spam_techniques