I installed apache and php in apache mod_chroot and to simplify the sendmail setup, i used mini_sendmail and everything works using php mail().
However, I use your plugin si-contact-form, and found it not working when I had "Custom E-mail From (optional):" enabled.
The apache error is:
usage: /usr/sbin/sendmail [-f<name>] [-t] [-s<server>] [-p<port>] [-T<timeout>] [-v] [address ...]
it didn't work with wp_mail nor php mail().
I solved it myself by modifying your code by removing the space behind the '-f' that is passed to mail() function:
so, this line (and there is another similar lines):
@mail($mail_to,$subj,$msg,$header_php, '-f '.$this->si_contact_mail_sender);
but mini_sendmail only works without the trailing space, so i chose to remove the trailing space behind -f.
what do you think?