imip inbound pop3 + mail w/o icalender attachment spams organizer

[dphi]

If there's an incoming mail that triggers: [txdav.caldav.datastore.scheduling.imip.inbound#warn] Mail gateway didn't find an icalendar attachment in message [txdav.caldav.datastore.scheduling.imip.inbound#warn] Mail gateway forwarding reply back to organizer

and pop3 is used, then

• this mail is forwarded every $polling_interval to the organizer, and
• the mail keeps sitting in that inbox

Suggested solution: Make https://github.com/apple/ccs-calendarserver/blob/b736fd0248ee42ac7a3b7d2af7fb3a55962d39ba/txdav/caldav/datastore/scheduling/imip/inbound.py#L190

a configuration setting.