It's been causing some major confusion recently, did you know that Steam now blocks most major email address' in their chat program?
After talking to a friend of mine and asking for his email, I received a message from him saying "http://gmail.com." At first I thought he was trying to troll me so I went along with it, later on after some more frustration we realized that Steam chat was actually blocking what we were sending to each other.
As you can see above, on the left I sent him a few different test emails to check exactly what is blocked. His result is on the right, showing that all of these email address' have been effectively turned into URL's. For those who don't keep up with Steam updates and patch notes, this little bit of news may cause you some headaches in the future.
This change is likely due to a massive rise in scammers, often linking emails and various URL's to users in order to gain access to their accounts and expensive skins located within.
Another interesting note is that '@xtra.co.nz' was not blocked - showing that it may be some sort of '@domain.com' automatic blocker with a few extra common ones thrown in.
If you're really in need of sending your email over steam, try typing it as "name at domain dot com," for example, "johnsmith at gmail dot com."