And I am still unable to find a bugreport for the problem (I searched for LMTP but all bugs I found are unrelated).
I have attached 2 screenshots (from ZCS NE 7.1.1 p2). lmtp_conn.jpg shows that the number of LMTP processes is going to the max. /var/log/zimbra.log starts showing that Postfix can not connect to LMTP:
I think that happens when the number of LMTP Processes is maxed out and all running LMTP Processes are busy. Question is: why are the LMTP processes "hanging"?Code:refused to talk to me: 421 zimbra.server.com Zimbra LMTP server closing connection; service busy
The second screenshot imap_conn.jpg shows the current number of imap connections. The peeks are at the same time (the screenshots have different scaling).
I have seen much more imap connections to that server, so I doubt, that IMAP is really the cause of the problem...
My assumption is, that LMTP can not put the mail into the mailbox for some reason. That causes new LMTP processes to be spawned until the max. of 20 processes/threads.
The only way fix this is to restart the mailboxd.
However: I found no indication in the mailbox.log why LMTP can not deliver the mail.