Magento 2 – JS Messages Not Translated

I have solved this myself.

Your module needs a dependency to Magento_AdminNotification.

Then you create a model which implements MessageInterface, like this:

class YourMessage implements \Magento\Framework\Notification\MessageInterface
{
  public function getIdentity() {
    ...
  }

  public function isDisplayed() {
    // write code to decide if this message should be shown or not
    // return true to show it, false otherwise
  }

  public function getText() {
    // output a static text or dynamically generate one
  }

  public function getSeverity() {
    // if you return self::SEVERITY_MAJOR the sticky box on top will be empty
    // if you return self::SEVERITY_CRITICAL the sticky box will always show your text
  }
}

That's really all there is to it. I spent so much time looking for the place in the code where these messages are triggered, because I didn't notice that they trigger themselves by using the isDisplayed() method.

You can get inspiration from other such messages used in core:

\Magento\AdminNotification\Model\System\Message\Baseurl \Magento\AdminNotification\Model\System\Message\CacheOutdated \Magento\AdminNotification\Model\System\Message\Security

In my opinion it's not possible. At most you can try to check if the domain in question have MX record and try to connect to its mail server. Even that won't guarantee it is in working condition though.

You absolutely can't check if a particular e-mail exists on it in some standartized way as there are many approaches to masquerade and aliasing that many server employ. Servers can and will report in SMTP exchange non-existent addresses as valid for many reasons in both VRFY and MAIL/RCPT. The only definite answer you can get is that e-mail is invalid if it is rejected by MAIL/RCPT, but being accepted is not definite proof of it being valid, as it can be rejected down the line of e-mail processing. Abusing MAIL/RCPT without actually sending anything can also lead to you being blocked.

If you want to verify user-supplied e-mail, your best bet is to send confirmation letter there.

You also should review if you really need confirmed working e-mail at all.