I know, it's an old issue, but it's affecting my sites. Yes, siteS.
I have one hosting and with two wp installations, one for the main domain and the other in the virtual domain (in a sub-directory of the main site)
None of them work.
They are different wp versions: 2.9.2 and 3.2.1
Both use its own database.
I called Godaddy but they say they tested the php with a php script they copied into my sites and the emails are sent fine.
I tried everything I found in these forums.
I tried different emails, all existing ones, no success.
Spam folders, server logs, removed all my functions, changed theme…
Finally, I added a redirect RIGHT before the $phpmailer->Send(); line in wp_mail() function, in includes/pluggable.php, sending the $headers variables as URL query, to a function in ANOTHER server I use to test and display/dump the post/get variables, like
header ("Location: http://somesite/viewpost.php?" . $headers);
…and it doesn't get redirected, so I assume the wp_mail() function is not even executed.