spam blocker problem

In the past two days my email messages have been getting blocked by several friends' providers (earthlink and mindspring addresses. One of them keeps blocking my emails, even after I've filled out the form three times. It seems that their spam blockers keep generating the same request form to be filled out in order to approve me as a bonafide correspondent. I spoke to one of my friends and he has confirmed that my yahoo address is in his address book, so I'm not clear about what is going on or how to fix it.
Thanks for any help you can provide.
William