In our case when there was an issue with the external MTA/SMTP server and Zimbra couldn't communicate with it this would basically "hose" Zimbra, and eventually tomcat would die. At that point the users would have issues regardless of who they were trying to send to.
I don't want to lead you astray, but the fact that you are getting the same error messages with nothing apparently wrong with your Zimbra server seems very much the same as the problem we experienced. If possible I would look at logs and whatnot on your external MTA server to see if it gives you any clues about what is going on. |