Sendmail won't accept emails from this Forum, because the address BT Community Forums <email@example.com> is from a hostname, community-mail.bt.com, that doesn't resolve in public DNS.
Fix for sendmail users, is to work around BT's DNS failure by adding this line to /etc/mail/domaintable and restarting sendmail:
This tells sendmail to treat mail from community-mail.bt.com as if it was coming from bt.com, a hostname that does resolve in DNS.
Why would you want to send emails from the forum and how are you doing so in any case?
When I registered for the Forum I had to click on a link in an email the Forum sent me to validate my email address.
But the email never arrived, so I couldn't click on the link!
Looking in my server logs, I saw:
reject=553 5.1.8 <firstname.lastname@example.org>... Domain of sender address email@example.com does not exist
My server was, quite correctly, refusing to accept email from a hostname (community-mail.bt.com) that doesn't exist.
I've managed to work around the problem by modifying my server, as specified above, to fix BT's missing DNS entries.