As a FreeFlarum guy, I can confirm that the test mail button was disabled to prevent spam. By default, there is no rate limit applied and with some scripting, it would be possible to send a lot of emails within a small time frame and crash our mail system, or at least make other mailbox providers flag us as spammers.
Since I don't know PHP, I did some copy-pasting and permanently throttled the test mail route. I don't know how to make it temporary or at least change the error message.
The only way to test the mail is to request a password change E-mail, warn yourself or simply trigger anything that sends mail at your forum (other than the test mail button, of course).
So, if test mail fails (which it always does), it doesn't mean that your setup is broken. We provide every forum with a working mail system for free (from our server).
I am thinking of changing this limitation later because many people mistakenly think that their mail setup is broken. If people would really want to spam mail on behalf of our server, they would find a way already
