Some e-mail systems, notably Earthlink, employ a challenge-response system whereby when you send a user an e-mail you get a response asking you to fill out a form to allow your e-mail to go through.
This is improper behavior because spam filtering is your problem (just like I filter my spam) and you’re pushing your work back on me.
It becomes an n2 problem if everybody employes this kind of system, so it’s not a scalable behavior. Supporting this kind of system creates a moral hazard, so, if you’re using one of these systems you’re not going to get mail from me.