On an Ubuntu installation, the php sendmail function will send out email as the apache user www-data@servername.domain. This can make the email flag as spam on some mail servers. To get around this edit your php.ini and restart apache.
Edit the php.ini
sudo vi /etc/php5/apache2/php.ini
Edit the following string:
sendmail_path = '/usr/sbin/sendmail -t -i -fno-reply@yourdomain.com -Fno-reply'
Now restart apache:
sudo service apache2 restart