This issue has been reported previously on the Webmin mailing list at SourceForge (https://sourceforge.net/p/webadmin/bugs/4517/?page=1 and https://sourceforge.net/p/webadmin/discussion/600155/thread/3717d120/?li...) but none of the reports have gotten working solutions, so I thought I would report this again on the Virtualmin site as I am now experiencing this bug for myself.
We run a cluster of four Web servers, each running Webmin (and one running Virtualmin). I have Postfix configured on the Virtualmin server, and have secured it by requiring authenticated mail clients to connect to port 587 and use TLS/STARTTLS together with SASL authentication before they can send e-mail.
I am trying to configure Webmin to automatically check for and install software package updates on the four Web servers I have, and want it to send e-mail notifications when this is complete, as well as send e-mails about scheduled backups and other related tasks. On the Virtualmin server, I went into Webmin > Webmin Configuration > Sending E-mail and chose to send via the local mail server command, which worked flawlessly.
However, I haven't been able to get Webmin to send e-mail on the other three servers running only Webmin. Because there is no local mail server on those machines, I select to send via remote SMTP server from Webmin > Webmin Configuration > Sending Email, then check the "use SSL encryption" box, change the port number to 587, and put in my authentication details, selecting LOGIN as the authentication method. But when I try to send a test message, I just get "SMTP command failed."
When I send the same test but uncheck "use SSL encryption" (still on port 587), I get the following: sending failed : SMTP command mail from: email@example.com failed : 530 5.7.0 Must issue a STARTTLS command first , line 4.
I can send e-mail in Roundcube, Apple Mail for macOS, Apple Mail for iOS, and Outlook but cannot send via Webmin.