As seen in T315 we should implement a sender blacklist.

This is required in order to be able to effectively create Tasks from an @linux.it email address. Actually it seems that our Linux.it Help Desk email address must be registered in mailman and so it receives all the emails from the "soci" mailing list.

Also, this will prevent mailman information disclosure, like the monthly password reminder, or anyway the manual password reminder.

This may be also useful to ban some spammy users as well.