451 4.0.0 Temporary message delivery failure

[glennbtn]

Hi All
We have a few servers running Ubuntu 12.4 with Zimbra 8.6.0 and patch 6 installed. 
Random users are not getting random emails but are getting others. When tailing the log we see
said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command)) and the message then sits in the queue defered
I have done a fixperms as read somewhere this fixed the issue but no joy with us.
Can anyone advise please
Thanks
Glenn

[bsarandan]

We have the same problem after upgrading this weekend to 8.8.5_GA_1894.RHEL7_64_20171026035615 RHEL7_64 FOSS edition on a CentOS 7.2.
We tried to fix the permisions with zmfixperms -extended -verbose, we also reindexed some mailboxes, the ones that are affected.
The problems is very strange, some emails are kept in the deferred queue and other pass but only for certain mboxes.

Did you find the solution to your problem? Thanks.

[bsarandan]

We've activated IMAP NIO ( https://wiki.zimbra.com/wiki/IMAP_NIO ) and that seems to do the trick, no more deferred emails and no more errors.