When I choose to set the system to use SMTP I always get this error and mail is not sent (for any purpose).

Warning: Cannot modify header information - headers already sent by (output started at /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php:2948) in /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php on line 3526

Warning: Cannot modify header information - headers already sent by (output started at /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php:2948) in /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php on line 3528

Warning: Cannot modify header information - headers already sent by (output started at /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php:2948) in /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php on line 3529

Warning: Cannot modify header information - headers already sent by (output started at /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php:2948) in /homepages/33/d93032303/htdocs/Vienna/forum/includes/functions.php on line 3530

I know how to use the SMTP server at my hosting firm and I understand how to set up authentication and ports. The php mail function does work though, mails are sent and no errors occur.

I hope this is not a duplicate but I did search