Incoming mail is checked by SpamAssassin. Its settings are under E-mail Manager → SpamAssassin Setup.

The key choice is the blocking strategy:
SPAM Filters additionally block particular senders, domains or words.
Autoresponders reply automatically to every message sent to an address (e.g. "Thanks, we will reply within 24 hours"). Vacation Messages do the same, but only for a period you set - give a start and end date and the reply switches itself off after your holiday.
