CiviMail: AMaViS-filter only incoming mail

Explore whether there's an easy way (and if so, implement it) to make the outgoing CiviMail emails skip AMaViS, as big bursts of email can bring AMaViS to its knees in certain configs (and outgoing mail is not supposed to carry a virus nor to be a CiviMail material).

For example, check whether header checks can be used to turn filtering off: http://www.postfix.org/FILTER_README.html#dynamic_filter