FOr security purpose, mail to security at tikiwiki.org
From the place you receive an email from tw, did you check the headers. Perhaps you will see something like "header improprely encoded with \r\n" (I don't remember exactly the message. In this case go to admin-> general and change the end of mail line to LF or CR
Just to check: are you in 1.9.1 at least - because before a return path in the header was misssing
It was very hard to fix tw to be sure that the message it sends is not considered as spam. Perhaps there is still some detail tahn can be considered as spam.
Same emails forbidden at registration time -> something to be developped in tw.