I have no problem with such addresses.
What exact error message are you getting ?
How do you send the mails (webclient, ZCO, other way) ?
Outlook with IMAP or Connector - Have seen the problem in both configurations.
The error is a Failed delivery message returne to the client, almost instantly. It has happened with a few different users sending to unrelated domains. The only common thread is the a.b@domain address format that causes the failure.
As Bill suggested, I guess you have a postfix setup issue...
Networks added. thanks for the nudge.