Some SMTP servers take more time to respond to IMSS causing the default timeout to be triggered.
This happens when the timeout occurs while IMSS is waiting for the "." command to end the SMTP transaction. When the receiving SMTP server fails to send the acknowledgement after the seconds defined by the "IdleWaitingSecond" parameter, IMSS considers it as connection time out and moves the mail to the retry folder for resending. IMSS keeps on resending the message based on the specified retry period until it succeeds or until the number of retries is met.
You will keep on receiving the same mails if the server continues to fail in sending acknowledgement response to the data session before the timeout is triggered.