Messages are not being processed by the MTA from the SMTP inbound queues



Messages destined for valid mailboxes on the server are not being delivered and are sitting in the SMTP inbound queue.  Or the message command files or message files are being left over and being left stranded in the queues. Restarting the MTA service causes the queues to be processed again.



The causes of this issue are varied and are as follows;

  • Malfunctioning Pick Up Event is not releasing the lock on files or not deleting command files or message files.
  • Plug-in Anti Virus program is failing and not releasing locks on message files or not deleting command files or message files.
  • Resident Anti Virus program is running and not excluding the MailEnable queues directories.  This can cause concurrent file access issues and not allow MailEnable or a pick up event to delete the message files.
  • COM object pick up event is failing this is most likely linked to the Plesk utilization of Spam Assassin.

Isolating the cause



In all cases the first step is to turn off the pick up events and AV to work out if that is the reason for the messages being backed up.  Here are the steps for disabling each option you may like to complete each option separately to work out which is giving you issues;

  • Stop and start the MTA and SMTP services to ensure that all messages that can be delivered are.
  • Stop the SMTP service
  • Check to ensure that all the messages that can be picked up by the MTA service are done so.  The best way to check this is with the Activity Monitor that is available in the MailEnable program group System Tools.  Open the monitor and ensure no more messages are being delivered to the Post Office Connector.
  • Stop the MTA service
  • Delete or move the message files out from the following locations, this will ensure after the plug-in's are disabled that you are left with a blank queue to ensure the issue is easily recognized as reoccurring or resolved;

Program Files\MailEnable\Queues\SMTP\Inbound

Program Files\MailEnable\Queues\SMTP\Inbound\Messages

  • Stop the Plug-in's, you can stop them all together or if you want to determine which one is causing the problems then you can complete these steps for each of the plug-in's below;

Stopping the third party MTA Pick Up Event


  • Navigate within the MailEnable Administration Console to: Servers > Localhost > Services and Connectors > MTA.
  • Right click on the MTA node and select "properties".
  • Untick the box for "Enable pickup event".
  • Click Apply and the Ok.
  • Right click on the MTA node and Stop and then Start the service.

Stopping the third party COM object Pick Up Event (Mainly Plesk users)


  • Open Windows "Regedit" and navigate to the following location:

For 32bit Windows: HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Agents\MTA

For 64bit Windows: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mail Enable\Mail Enable\Agents\MTA

  • Locate the key name: Pickup Event Class Enabled
  • Change the Decimal value to "0" to disbale the COM object.
  • Navigate within the MailEnable administration console to: Servers > Localhost > Services and Connectors > MTA
  • Right click on the "MTA" node and select "Stop" and then "Start" to restart MTA agent. 

Stopping the resident anti virus program


This varies between the antivirus applications , the important thing to do where possible is exclude the MailEnable folders from being checked by your resident Antivirus scanner.  If this is not possible then it should be disabled altogether. Running resident antivirus scanners on a mail server can cause many problems due to the high number of files being moved around by the mail server and subsequently depending on the actions requiring access or blocking access to these files.  In regards to the antivirus, access to these files is required by the MTA and its plug-in's at all times while passing through the service.  This is where the problem of resident scanners inhibit the MTA, while a message is being accessed by the MTA, the resident notices this file access and then does its job by locking the file and doing its own scan while doing this excluding all other programs from accessing the file.

Depending on when the file is locked and how long the scan takes, this will affect the MTA making the occurrences seem very ad hoc some results of this are below;

  • some messages with viruses will pass through,
  • some will be detected as normal
  • blank messages can be delivered to mailboxes
  • Messages can appear to get lost in system (really the resident is quarantining or deleting the message)
  • In rare circumstances the MTA can crash or messages can become corrupted.

Disabling MailEnable Antivirus scanning


  • Navigate within the MailEnable administration console to: Servers > localhost > Extensions > Message Filter.
  • Click on "Message Filter" to list the options in the right hand pane window.
  • Right click on "MailEnable Antivirus Filter" and select "Disable".



As always ensure you are running the latest version of MailEnable which can be downloaded from the MailEnable web site.  For an issue like this it is imperative that you are running the latest.


Once you have isolated the problem then the resolutions are as follows;


Pick Up Events:

If you are the developer of the pick up event then ensure that the pick up event(s) are releasing any files that that are being locked for a completed action.  If the pick up event is deleting then ensure that the delete is occurring.  If the pick up event is supplied by a third party then you should contact them to either contact MailEnable for support or to diagnose and fix the issue.

If you are not sure what the pick up event does or do not need it then you can leave the event disabled.  For Plesk users the pick up event and the COM component are part of the Spam Assassin plug-in to their product so if this is not being used then you can again leave it disabled and use the MailEnable filtering for spam protection.




You may need to lower the threads on the MTA for your program to run effectively.  The best way to prove this is required is by lowering the threads to one to see if the problem still exists if it does not then you can gradually increase the threads until you determine the best setting for your plug-in.

Product:MailEnable (All Versions)
Class:BUG: Product Defect/Bug
Revised:Wednesday, April 18, 2018