Warning : Cannot add header information - headers already sent by (output started at /your/site/www/mail/config.php:113) in /your/site/www/mail/signup.php on line 53
If you are receiving the message above, here is the solution.
Some (FTP) clients automatically send .php files as Binary as opposed to ASCII which can alter formatting.
What you need to do is remove all extra characters from the end of your config.php file after the closing PHP tag:
?>
The characters will likely be "invisible" spaces or line breaks. Simply hit delete a few times with the cursor after the last bracket, save the file, and re-upload it.