The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
Guess we have to use gmail then. I tested 3 "Microsoft" emails. Hotmail.com and .fi and outlook.com. Anyhow the mailserver/domain seems to be on somekind of public blacklist, or there is dns issues, since it gets rejected by private servers also(rspamd).