Some forums use an online service (website) where you can enter an email address and the service will validate the email address using a series of tests. Unfortunately, the last test is a test of the email service itself, not the individual email address. As far as I've been able to tell, that last test always fails-- that is, gives a failing grade-- to some of the free email services, such as aol.com and yahoo.com. Some forums will reject your membership application and flag you as a spammer-- that is, add the email address you tried to register with, as well as your ISP address-- on an online anti-spam database site, even if you've never sent any spam, simply on the basis of that last test coming back with a failing grade. Personally, I think this is unconscionable, because that final test is-- in my humble opinion-- questionable at best, since it tests the email service and not the specific email account. So basically, if you try to register to forums that use that validation service-- and who make their final judgment on the basis of that final test, despite the fact that your email address passed all of the other tests with flying colors-- then your email address will be entered into that database as a spammer.