I am not getting any emails from Kanboard, even though if I send a comment or a card by email, Kanboard gives a message saying it was successfully sent.
I have tested that my server will successfully send an email via PHP mail() with a test php script.
So after a lot of testing, I discovered that PHP mail fails when there is an invalid email address in the headers. Since I am using PHP 8, and the PHP constant PHP_EOL has changed between php 7.4. and php 8.x., I followed the advice here and it worked!
May I ask what version of Kanboard you have installed, Miss Lara? Because the version of Kanboard I have installed is 1.2.41, and I encountered the same issue as you when using SMTP to send emails.