Actually its mostly at&t and its sub owned stuff like sbcglobal that wont accept emails from forums 90% or more of the times. I put in tickets, they allow them for a day or so then its back at it again. Complaining to your net provider helps a bunch believe it or not.
Emails go out 100% of the time. Just some places wont accept them because of where they are hosted or its considered "spam".
What usually starts it with an email provider is someone signs up with a site and then doesnt want email from it anymore and in place of unsubscribing they flag the mails as spam or report them as spam.
But if its at&t or sbcglobal they are just a pain like that.