For last few days users in our organisation started complaining about non receiving of the mails from external senders. i.e. if any person over the world sends mail to any of the user in my zimbra server, mail is not received at all (inbox/junk) and not bounced to sender also. In such case, sender has the mail in his sent folder. This happens to mails from gmail more frequently, but from other domains also the problem persists.
Can anybody please guide me on how to troubleshoot the matter.