IIS SMTP TLS and SSL auto negotiate issues


  • I am running GFI mailessentials on a DMZ server.

    I am using IIS SMTP for this at the moment as I migrate to Exchange 2016 and re-do the server with Exchange 2016 Edge on it.

    But at this time when I enable outbound security TLS communication, all domains we communicate with that do not accept TLS start sending error:

    Message delivery to the host '' failed while delivering to the remote domain '' for the following reason: The remote SMTP service does not support TLS.

    It seems that IIS SMTP is not auto negotiating the connection. When I enable TLS, it only uses TLS. I have tested this many different ways. Yes certificate is installed, as we have tested with 30+ different domains the difference with TLS enabled, and TLS disabled.

    Anyone run into this and know how to get this to auto-negotiate vs it just requiring TLS vs SSL.



    Monday, June 4, 2018 2:29 PM

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    Monday, June 4, 2018 6:43 PM