This applies only if the SMTP server runs on the same host as Gogs. Use case: my mail server has a lot of checks for legitimate users, but OTOH this would require a huge burden for services (which usually do not need a full account).

Therefore, using "sendmail" (quotes, because a lot of other MTAs like Postfix offer a sendmail-compatible interface) makes sense (of course, only if the system running Gogs and the mail server are one and the same) and doesn't even require a connection to the SMTP server.