Resolve SBS 2008/2011 Pop connector issues, Emails Rejected Due to Protocol Errors

Have been facing this annoying issue while working with one of the SBS servers. pop connector keeps throwing up errors on the event log and emails will be not downloaded on to any affected accounts

Error-

 “One or more (X number of ) e-mail messages in the POP3 mailbox account ‘[email protected]’ on the POP3 server ‘mailserver.com or IP’ have invalid header fields. Because of this, the messages cannot be delivered to the Exchange Server mailbox ‘[email protected]’ in Windows Small Business Server. The messages are still on the POP3 server. To resolve this issue, connect to the POP3 mailbox account, and then manually retrieve or delete the messages"

Why this is happening

Affected mail boxes have NDR's or any spam / auto generated emails with invalid mail headers

Reason for rejecting the emails -

According to RFC rules, a valid address should always have a "@" in it.
e.g.
<[email protected]>.

However, sometimes non-compliant mailers use addresses like <MAILER-DEAMON>.
In this case <System Administrator>

When downloading emails via POP connector, Exchange tries to verify the "return path" for the mails by reading the email headers. If the return address is malformed it rejects the email and the Exchange will end the session with a transient error (4xx). The pop3connector recognizes that the error is transient and retry again at the next scheduled download period
Since exchange didn’t accept the message the POP connector doesn’t delete the emails from the SM pop account.


Solution


Quick Fix:

Until you do the change to get the mail flow working again. you can simply

• login to the POP account 
• Remove or move the NDR's or any other auto generated mails
• Sync the POP connector

Permanent (long term) Work arounds:

There are two work arounds for this

FIX 1 (resolution)
 

In most occasions, as a resolution, Modify the ReceiveConnector to append the local domain name for any malformed return addresses. so the email will be considered as an internal email. So tweak your spam filters and Mail AV accordingly


Open a Elevated Exchange Management Shell:


Set-ReceiveConnector -Identity ($Env:computername + “\Windows SBS Fax Sharepoint Receive ” + $Env:computername) -DefaultDomain $Env:UserDNSDomain



In some It environments above fix wont be allowed due to IT security policies so the following will be your only option to tip toe around this problem



FIX 2
 
Increase the "MaxProtocolErrors” property value of the  Windows SBS Fax Sharepoint Receive connector

Open a Elevated Exchange Management Shell:

Set-ReceiveConnector -Identity ($Env:computername + “\Windows SBS Fax Sharepoint Receive ” + $Env:computername) -MaxProtocolErrors 500


This increase the number of protocol errors the pop connector will ignore to 500, not a permamant fix but POP connectors are not usually used as a permanent solution either.

Hope this information was useful. It sure did save my behind.