Back when my mind was becoming slightly untethered trying to route email directly from my server, I came across one email relay that was willing to forward me my email from their servers for free but I had to guarantee things were in working order; which is completely fair of course.
It was then when I learned that a Telnet client besides of its normal outdated SSH-like functions, it also works to check on open SMTP ports. Telnet, though not immediately obvious, still comes with Windows.
In the Server family, it’s available as part of Windows’ Features. To install use Server Manager and add the feature to the server you wish.
The faster way is with an Administrative PowerShell window; execute
Install-WindowsFeature -Name Telnet-Client
Yep… Each passing moment Microsoft and Apple lock things more
and make it more difficult for you to find settings that might reduce telemetry or allow you install things where they don’t get a cut when the purchase wasn’t made through their stores for your security.