The issue occurs because the receiving MTA is slow resulting to timeout issues experienced by the IMSS server when processing outbound email messages.

To resolve this issue:

1. Locate and open the tsmtpd.ini file. By default, it is under C:\Program Files(x86)\Trend Micro\IMSS\config\.
2. Go to the [SmtpClient] section and look for the "IdleWaitingSecond" parameter.
3. Change the value of the "IdleWaitingSecond" parameter to "60".
    

   The value should not exceed "90" for best delivery performance.
   Increasing the "IdleWaitingSecond" parameter can affect the delivery performance because the delivery threads will remain in a longer waiting state. Messages pending for delivery will have to wait in queue before the next thread becomes available.
4. Save and close the file.
5. Restart the following services:
   • Trend Micro IMSS SMTP Service
   • Trend Micro IMSS Scan Service

If there is an increase in the Delivery Queue, you may modify the value of the "MaxDeliverThread" parameter to make up for the longer timeout:

1. Open the tsmtpd.ini file and go to the [SmtpClient] section.
2. Look for the "MaxDeliverThread" parameter.
3. Set the value of the "MaxDeliverThread" parameter to "24".
    

   This is the recommended setting, which you can gradually increase as you monitor the performance of the server.
4. Save and close the file.
5. Restart the following services:
   • Trend Micro IMSS SMTP Service
   • Trend Micro IMSS Scan Service