STARTTLS Server SMTP with cleartext and STARTTLS client support
Published May 02, 2019
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So just FYI, while this did work on a test client (I was using Nessus SMTP testing) other clients may not work because of the way they seem to handle sending commands. For example: Elastalert seems to send EHLO and that immediately after send the mail from: address, but a cleartext client needs to re-EHLO; while O365 does appear to accept this slight mis-ordering, when the client gets the command for data (354) it doesn't return any. So, you milage may vary.