Two possible issues:
1) I have temporarily increased my message size limit in the admin UI Global Settings/MTA/Maximum Message Size to 15MB in order to import some of my converted users' inboxes, with instructions that they have to clean house soon. I'd like to ratchet that limit back down in order to prevent new oversize messages from coming in. If I do so, does Zimbra care about the now-over-limit messages that are already in the message store, or does it only apply that test to incoming messages?
2) During the conversion phase I also have fetchmail pulling down some users' pop accounts. I presume Zimbra will simply reject messages that are over-limit, but what happens then? Do they stay in some sort of fetchmail limbo, do they /dev/null, or do they bounce somehow? And is there any way for me to monitor or control this?