I have set up an SMTP-Account over Office365 in the Global Settings. It did not work. After debugging, I found out that Office365 refuses to send the mail because the user is not authorized to send from another "from"-address other than his own (obviously).
Based on the discussion here: https://github.com/PHPMailer/PHPMailer/issues/1250 for PHPMailer, the setting $mail->Sender should not be set by default.
After changeing the corresponding line at application / helpers / common_helper.php on line 2281 from
$mail->Sender = $senderemail;
to
$mail->Sender = $fromemail;
or completely removing it, everything worked as expected.
$senderemail contained my own email-address, $fromemail contains the address set in Global Settings.

For me, this change worked.
For you, I would advise that you check which way you want to have it solved.