Yeah, but we're not talking about obscure email clients here, this is Outlook 365, Outlook 2021, Outlook 2010, iphone's mail app, Outlook mobile, etc; they ALL seem to have the same issue. MS even recommends disabling SMTP Auth on Exchange
https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission.
If there's something I'm missing, by all means I'd love to find it; For example, checkTLS.com responds with the following EHLO from gmail:
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
Gmail does accept SMTP Auth on port 587, and none of the aforementioned clients have an issue connecting (but there is no EHLO AUTH code returned). if I uncheck 'Only Allow Secure Login' from the Groups entry for my users, and I try to connect to SMTP w/o authentication, it still fails in the same way on the client. HOWEVER, if I ALSO disable SMTP Authentication entirely, it will connect ok:
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Accept Connection], Details=[Port 587]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Received Hello], Details=[Host=MACHINENAME]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Start TLS]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Received Hello], Details=[Host=MACHINENAME]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Received Sender], Details=[user@domain.com]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Received Recipient], Details=[user@domain.com]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Start Mail Transaction]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Complete Mail Transaction], Details=[From Host=MACHINENAME, Size=1 KB, From=user@domain.com, To=user@domain.com]
Tue, 26 Jul 2022 16:34:33 -> 192.168.1.254 -> Success: Action=[Close Connection]
So I think the issue lies somewhere in checking 'Disable AUTH reporting' while still having SMTP Authentication enabled that's causing the aforementioned connection problem. Given the option, I'd still rather be able to use SMTP authentication rather than having an open relay, if nothing else b/c AMS doesn't support stuff like OAuth yet...