Which e-mail servers? I think there is some confusion here.
We have a form on our page that asks for an e-mail address. When it is posted, it is sent to our client's e-mail address unless you enter any of the domain extensions I've listed.
We initially thought this was because our client's e-mail address/server might be blocking these extensions so we changed the destination e-mail address and nothing changed. This indicates that it's nothing to do with the receiving e-mail server, does it not?