Robert_Charlton - 8:01 am on Feb 8, 2010 (gmt 0)

Even when I reply to an email?

Often especially when you reply to an email. I've run into a number of situations where emails would get kicked back because the return addresses had been set up as I described above....

UserName@example.com <username@example.com>

That "@" character is an "illegal" character in user names, but apparently it doesn't affect senders... only those who reply to it.