When registering for forums, I see this error: One or more of the fields entered was marked as spam.
We use Lithium Studios Antispam ACP module which tracks email addresses and IP addresses from which spam has originated from in a global list. It integrates with https://akismet.com/ and with http://www.stopforumspam.com/
Unfortunately if you receive this error, it appears that your IP address, your username, or your email address was marked as non-trusted by the service. You can check the above URLs looking to see if your IP or email was flagged, and why. Unfortunately we have no ability to change their identification as "spam" from our end.