I have Exchange 2013 CU2. Recently, one of our users received an e-mail message with sender's addressuser_a.b.@domain.com. It was impossible to reply to that address because Exchange refused to accept it:
----- The following addresses had permanent fatal errors ----- <ADDRESS> (reason: 501 5.1.3 Invalid address)
It was possible to send the message if the second dot was removed (i.e. no more ".@" combination). Of course, it could not be delivered anyways (no such mailbox).
I've conducted some experiments and found that both my mail server running Sendmail and Google mail system accept that address and deliver messages to it.
Why does Exchange behave in such a way?
