Recently, I did an upgrade from phpBB3 version 3.0.0 to 3.0.5. I noticed that there was an error when people were signing up. This was the error message I got in the phpBB error logs.

[phpBB Debug] PHP Notice: in file /includes/functions_messenger.php on line 446: mail() [function.mail]: Bad parameters to mail() function, mail not sent.

I had determined that the error came from the mail_encode function from line 446 in the /includes/functions_messenger.php file. My temporary solution was to remove that function. I will look into it further to see if I can find a proper fix for it.