SYMPTOMS

The symptoms are varied for this problem but here are the three most common indicators:

1.  You receive a bounced message back to the client with the following error message;

MailEnable: Message could not be delivered to some recipients.
The following recipient(s) could not be reached:

   Recipient: [SMTP:xxxxx@xxxxxx.xx]
   Reason: Remote SMTP Server Returned: -

Message headers follow:

2.  SMTP logs in the MailEnable Activity log show a QUIT command after a RCPT TO without a DATA command.  In the debug logs you would most likely see a disconnection error like the following:

ME-E0070: (recv) socket [XXX] error during [RCPT] command from host xxx.xxx.xxx.xxx Socket was disconnected - Error: (10060)

3.  If you disable the MailEnable SMTP Blacklisting the server will accept messages being sent from the same server in the same way.

CAUSE

This problem occurs because the receiving server is doing a Blacklist check on the sending servers IP and this check is either taking too long or failing.  While these lookups are being done by the receiving server the sending server has reached it's connection timeout limit and dropped the connection.

RESOLUTION

The best way to resolve this issue is to find out which Blacklist(s) are failing or taking time to return the checked results and then remove or reconfigure the list to ensure that it is returning results in the few seconds that is normal.  There are two ways to work out which list is causing problems the first is using a trial and error method by removing all blacklists and then re adding them one at a time until the sending timeout failure occurs again.  The second way is to enable the SMTP advanced logging and then check the debug log to find out which list is causing problems.

To enable the advanced debug logging for the SMTP service you need to add the following registry key and give it a value of 10 (decimal):

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Connectors\SMTP]
"Debug Logging Level"=dword:0000000a

After this is done you can restart the service and view the extra logging in the SMTP Debug log.

WORKAROUND

The other way that may help in this situation is to increase the timeout value for the sending servers SMTP service.  This increase in value would give the receiving SMTP more time to complete the Blacklist checks.  This fix will work in the situation where the Blacklist is simply taking too long to return the results but it is still recommended that you try to where possible diagnose what is wrong with the Blacklist(s).

MORE INFORMATION

How to Backup, Restore and Edit Microsoft Registry.