I know that this question was asked before.
When I call the PHP mail() function, it returns true.
I checked my php.ini (I'v running CentOS):
SMTP = localhost smtp_port = 25 sendmail_path = /usr/sbin/sendmail -t -i mail.add_x_header = On
I read in a forum that I have to install sendmail. So I installed it. Now sites with a mail() function doesn't load anymore. So I removed sendmail, and the mail() function returns true again, but doesn't send the mail.