I am having the same issue as strandwitz, but slightly different.
Running 6.4, and when a new user registers, my forum emails them their activation link (been doing this since it became an option).
The email is no longer sending out. I tried emailing myself a TEMP password as well...I never got that.
I can SEND myself an email, and I ran mailtest.php (someone posted this as a test out here)...but the forum does not seem to be sending any emails.
And I already checked for any blank lines in the filtered emails.
Any help?