When php's mail() function in includes/mail/UserMailer.php line ~433 is invoked, the first argument it receives is a MailAddress Object instead of a string.
This way the mail in sent, but the recipients are not treated correctly, and the mail produces an error in the mail agent.
I'm getting that through a command like:
UserMailer::send( $targetAddress, $submitterAddress, $subject, $this->text, $options );
I've checked that the UserMailer::send's parameters have their correct object type.