Hi Max,
the description tells one more important detail: This issue only affect sites that leave $CFG->smtphosts empty.
How did you configure your instance? Look at its value in the proper section or search it using the search box in the Administration block.
In case you'll have it empty, just drop two new lines in your config.php file e.g.:
$CFG->noreplyaddress = 'noreply@<yourdomain>';
$CFG->supportemail = 'support@<yourdomain>';
HTH,
Matteo