Sometimes you noticed your mail takes a bit longer than usual to get to the receiver. This issue may caused by spamming activities or your zimbra services are not working correctly.
If this issue caused by zimbra services, just restart the services by using zmcontrol restart. If the problem is still not resolve you may try to delete the deferred messages by following below steps.
Step 1: Check the Deferred Messages
Before you delete you may need to check the deferred messages first. To check deferred messages, use below command. Make sure you are login as zimbra user.
|[zimbra@mailsvr ~]$ sudo ~/libexec/zmqstat
Step 1: Delete the Messages
if you really want to delete/flush all stuck messages in mail queue that have been deferred, issue the command below;
|[zimbra@mailsvr ~]$ sudo /opt/zimbra/postfix/sbin/postsuper -d ALL
postsuper: Deleted: 83378 messages
Now check again the queue messages by follow the step 1.