It sounds like their ISP has a policy against Commercial Mass Mailings in order to alleviate SPAM.

If the mailing is legit and opt-in..meaning the recipients have established a business relationship with them and have knowingly and willingly requested to receive information from them, their ISP should be willing to work with them on terms under which they can perform this mailing.

If however, the above is not true and you are looking for a way to circumvent their attempts at preventing SPAM from originating within their network you are probably better off not doing so.