Reply #7 –
Finally, I found out the problem in key generation (just in the last days I reentered this area).
Line 46 on Mail.subs.php seems to contain the problem (v. 1.1.8 ):
$maillist = !empty($modSettings['maillist_enabled']) && $from_wrapper !== null && $message_id !== null && $priority < 4 && empty($modSettings['mail_no_message_id']);When I remove the from_wrapper test, the key is generated and the mail arrives in the forum.
Of course, the problem may also originate from the call of sendmail() without the wrapper argument, resp. a configuration setting that is related to wrapper.
It can clearly be seen that the security key is on the bottom of every email, enclosed by square brackets (would be helpful information into the appropriate wiki page.