I have been tasked with setting up mailboxes for a FAX to e-mail system and we don't want to retain any messages older than a day for these accounts. The closest I've found to an answer is this:
"There is one safeguard in place. If zimbraMailMessageLifetime is set to less than 30 days, an interval of 30 days is used. All other attributes don't have a safeguard."
Which comes from here: Mailbox Purge - Zimbra :: Wiki
If my reading of that is correct, it sounds like no matter what I set these values to , the zimbraMailMessageLifetime cannot be less than 30 days. Is this correct, or is it possible to make it 1d?