Equisys technical notes

Technical guidance, explanations and fixes for our products

FIX: Multiple copies of the same fax received to user's SMTP mailbox



This Zetafax technical note applies to:

  • Zetafax SMTP Server Version 17


On rare ocassions, some or all user mailboxes who are set up to receive faxes or fax sent notifications to their email inboxes via SMTP may received more than one copy of the same fax message or notification.


This issue is caused by the Zetafax SMTP Server not receiving a final acknowledgement from the remote mail server when terminating the transmisison at the end of a message (an SMTP QUIT call is sent from the Zetafax SMTP Server and it expects an acknowledgement, but does not receive it and when it polls the remote mail server the next time, it receives a message that indicates the connection has already been terminated).

If the QUIT acknowledgement is not received by Zetafax SMTP Server, it treats the transmission as failed and retries up to a maximum of 5 times before giving up. 

Since the SMTP message data has already been transmitted and the QUIT call may well have been received by the remote end, the message may have been treated as successfully completed on the remote server, but not by the Zetafax SMTP Server owing to the lack of the final acknowledgement.


Hotfix - This issue was resolved by the addition of a configuration option in a hotfix, which is available by contacting Equisys Technical Support and quoting the technote number ZTN2147. This new configuration option adds an additional check that ignores the lack of an acknowledgement and treats the message transfer as successful if the connection terminated message is received after polling the remote mail server.

Note that the hotfix requires an addition to the registry, detailed in the readme for the hotfix.

Registry Editor warning

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Equisys cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved.


This behaviour was corrected in the hotfix detailed above.

Last updated: 23rd August 2021 (NT) 

Keywords: SMTP QUIT ACK 221

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