What do you mean exactly with a "sponsor" needed?
Some additions you haven't mentioned yet:
inbox.lv
spray.se
safe-mail.net
mail.be (same company as ContactOffice to my knowledge)
mail.su (could also be email.su, not sure by heart)
mail.ir (could also have become email.ir, not sure by heart)
mail.by
terra.es
web.de
zoho.com
t-online.de (used to be available also for non-customers of the ISP)
ice.is
Sorry if there is a wide list of countries randomly listed, just gave you some services that popped in my mind that you hadn't listed as yet.
PS1: some services that provide free subdomain/URL redirections claim to also offer webmail. Eg .de.vu if I'm not mistaken. The problem why I am wary of listing anything of these is that those services change TOS regularly, and that I'm not sure if that webmail isn't just using the servers of a webmail provider that you already tested (eg Everyone.net)
PS2: if you have any specific good or bad comments on any of those services, please keep us updated
