Why does the forum-software persistently refuse to send a confirmation-mail to my mail-account? Is t-online.de domain blocked for outgoing mails?
I already tried to resend the essential mail several times, but no success… 
And no rejection by sending to spam-folder, I’m sure!
Why…?
The only possibility was to choose another mail that redirects to the originally desired address - and swoosh, confirmation mail is received within seconds!
?
The same mail - sent to a spamgourmet.com-address and then forwarded to my t-online-address, is received within seconds without any problems.
In Addition, mail-filter is configured not to reject any mail, but to move suspicious or ‘as-spam-declared’ mails into a spam-folder.
But no mail appeared there…
André
Which unfortunately does not rule out, that your receiving mail server rejects the sending mail servers IP address for some blacklist reason.