"Call to undefined method PHPMailer::IsMAIL()"

Hi everyone. In my web forum, http://gencgonulluyuz.biz/ I have problems with e-mailling. In private messages, when sending someone a message, i m getting Call to undefined method PHPMailer::IsMAIL() in /home/genc/public_html/includes/sendmail_include.php on line 34" error. The message is sent anyway but, i think because of this error, the notification e-mail is not sent to the receiver. I reloaded the whole infusions directory to solve this, but its is still going on. Also there exist some problems about the e-mailing.
In registration part, although the one who wants to register enters his email correctly , "e-mail is not correct" error is seen. Again, in password reminder part, when requesting the password, given correct e-mail is not accepted.
What can be the problem with those?
Thanks in advance...