Hey there,
I had an issue where my Kubernetes/Docker container was unable to send emails through SMTP, because of the infamous “does not match the expected structure for a DNS hostname”.
It is due to the fact that the PostmanUtils::getServerName() method uses $_SERVER['SERVER_NAME'] as long as it is defined, regardless of whether or not the value is empty (string(0) "").
I fixed it by changing line 452 of Postman/PostmanUtils.php to:
if (isset($_SERVER) and array_key_exists('SERVER_NAME', $_SERVER) and !empty($_SERVER['SERVER_NAME'])) {
(adds an empty check on the value). It then figured the proper container hostname.
If this was on GitHub, I’d have done a pull request but… Well, it’s been a while since I’ve used SVN, so it would be nice if someone can add that to the repo so that my manual change isn’t overridden on the next plugin update 
It shouldn’t affect any existing install.