Warning: Technical babbling ahead! I just ran into the problem that my server can't send mail with PHP and exim. Exim worked fine for any other process, just PHP would not send anything. The solution was to adjust the sendmail_path setting in my php.ini: It defaults to sendmail -t -i, but the -i option causes exim to falsly wait for more input. For Debian, just set sendmail_path = /usr/sbin/sendmail -t and PHP will be able to send mail.