Found quite a number of bounced mails in Exchange server, why?

This is using MS SBS 2011 server, with MS exchange. The bounced mails issue have been encountered for the past few months. I went to the console > queue mail viewer, always found few queued mails with error code - 4.x.x, why? What went wrong and how to troubleshoot the problem?

Thanks!
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MichaelBalackSenior System EngineerAsked:
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Kent DyerIT Security Analyst SeniorCommented:
You should be able to open the message header in outlook or your attached mail client..  In Outlook, it is the properties of the individual mail and should show the detail of the message in question..
MichaelBalackSenior System EngineerAuthor Commented:
Hi Kent Dyer,

All these are outgoing mails queuing in exchange server.
jerseysamCommented:
If outgoing mails then could just be mistyped email addresses etc or recipients that had an issue.
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Pradeep VIshwakarmaCommented:
first check ur server reputation
MichaelBalackSenior System EngineerAuthor Commented:
Hi Pradeep_vish,

How to check the server reputation?
MichaelBalackSenior System EngineerAuthor Commented:
I checked the queue viewer and found few mail in queue with status - retry and error message:

451 4.4.0 Primary target ip address responded with: "421 4.4.2 Connection dropped due to SOcketError." Attempted failover to alternate host, but that did not succeed. Either there not alternate host, or delivery failed to alternate host.
Simon Butler (Sembee)ConsultantCommented:
That is a connection failure.
Primary cause of that is interference - SMTP traffic being scanned by AV, firewall, something like that. Could be the remote host has an issue, but it is a poor way of dropping the email.

Simon.

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MichaelBalackSenior System EngineerAuthor Commented:
You are right, those mails blocked by the security features of the firewall. After disabling these features, no more mail in queue.
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