Zimbra lmtp not delivering mails
Ours is multi-server installation of ZCS 7.1.2, with two ldap servers(master and replica) and one MTA/Mail store. Around 1500 users are using , the users have only web mail access(not using thick clients).
During some time the lmtp fails to deliver the mails to the recepients mailbox, and the incoming mails are staying in the mailq for longtime(at times it go beyond 2 hrs). After this time , some times after 2hrs or so Zimbra recovers by itself and starts delivering mails. At times even after 2hrs it will not deliver the mails , the mailq will pile up to huge numbers and it requires a restart of zimbra service to recover from this state and to start delivering mails.
We have an MTA agent configured for a different domain , the mails send from zimbra to that is delivered quickly during the mentioned situation.
In /var/log/zimbra.log is showing nothing unusual. During this time people are able to login to their webmail, search mails and read their mails with out any issue.
Please help if anyone has idea ,why this is happening.