Clarify "Enable TCP Authentication Automatically" setting

Short version: The "Enable TCP Authentication Automatically" setting should be clarified and altered. We discovered that we need to disable this setting in SMTP 1.0 to work with our mail relay server, but this took quite a bit of trial and error, since this option didn't exist in earlier versions of the module (we were using beta7 previously), and it's enabled by default now. We think a better description of what the option does and (crucially importantly) a suggestion for when it should be disabled would be helpful. An alternative would be to have this option disabled by default, or to have an "Auto" setting that keeps it disabled when using Port 25 with no encryption or authentication.

Backstory: Our group has been using drupal/smtp:1.0-beta7 for a while now, because of SMTP errors we received using all of the RC versions. We use a local mail relay server that allows whitelisted hosts on our network to send SMTP requests to Gmail, authenticating with a functional account granted by our university. In other words, if Drupal SMTP is set to send mail to our relay server (port 25, no encryption, no auth), it should just work. It stopped working quite a few versions ago, and we didn't discover until today that the culprit was the "Enable TCP Authentication Automatically" setting, which was introduced to the module and set to be enabled by default. It seemed to us that if it was suppose to use TCP automatically, but not necessarily for every request, that it would not use TCP if it was incompatible with the server. Instead, it failed with a not-super-descriptive error message.

Proposed resolution

There are a number of options that could make this more user friendly:

• Add some text to the description of that option which recommends you disable it when using Port 25, or no encryption, or no authentication, or whatever would prevent situations like ours going forward.
• Create a new "auto" option that would be the new default, which would disable it when using Port 25, or no encryption, or no authentication, or whatever would prevent situations like ours going forward.
• Return to the default operation from 1.0-beta7, with the option to change it if desired.

I can understand that many users are pointing SMTP directly to Google or similar services, and if no one has had this problem besides us, then feel free to call it an edge case and not worry too much about it. But this did affect the management of the roughly 70 Drupal 8 sites that we host, so I figured I'd at least submit a request.

Thanks in advance!