Ensure @<username> doesn't over-reach when sending notifications
    
    If someone comments in a ticket or a PR with a text that contains an
    email address, for example: foo@bar.com and the domain corresponds to an
    existing username, we do not want to notify that user.
    (Imagine if an `gmail` user gets created! :D)
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>