I also observe that some external links should be allowed, particularly to Wikipedia, but understand the need to keep spammers at bay.
I realize that many spammers are now actual people getting paid to disrupt the forum but its worth a try.
These include That the spammers are infinitely adaptable in being able to get their messages across.
Spammers almost exclusively use bogus sending addresses so will not see the error message.
No person or persons are willfully excluded, just dratted spammers!