This is one of those odd scenarios that should seem impossible to occur on the face of it and only believed when observed.
We have a client that utilizes an internal application of which the version they currently run does not support TLS for sending email. The Exchange receive connector through which the application transmits email has the "Transport Layer Security (TLS)" option selected on the authentication tab. The only other option selected is "Externally Secured." So, with this configuration, you would think the application wouldalways fail to transmit email through the Exchange server.
However, I observed with Wireshark that some workstations will successfully transmit email through the Exchange server using plain text (no TLS). But other workstations will fail to transmit email as they attempt to utilize a TLS connection.
I'm a bit baffled by the following:
1) If the receive connector is configured to only allow authentication via TLS, why is it accepting email through a plain text connection?
2) Why do some workstations initiate a plain text connection and some workstations initiate a TLS connection? I don't know if this holds in all cases, but in the two computers I tested, the one that failed to send email (because it was trying to utilize TLS), was a Windows XP computer and the one that succeeded in sending email (because it was using a plain text connection) was a Windows 7 computer.
I'm by no means an Exchange expert, so if anything needs to be clarified further, please let me know. I'd really like to answer the baffling questions above, because, as I see it, both computers are acting weird (one initiates plain text when we told it to only use TLS and the other initiates TLS instead of plain text for some unknown reason).
Thanks!