reject mails to buildbot at SMTP level

[sascha_silbe]

The buildbot user doesn't process emails, but receives a lot of SPAM. We should block it at the SMTP level.
We could also discard messages during delivery, but a) that won't mark the email address as not working anymore in the spammers database and b) would also silently discard any legitimate message that might have accidentally landed there (e.g. cron job output).