There is a section in PHP.ini called Modules, under that there is a section called Mail Function, i think. It's been a while. Here's a snipp.
[mail function]
; For Win32 only.
SMTP = localhost
smtp_port = 25
; For Win32 only.
;sendmail_from = me@example.com
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
;sendmail_path = "C:\Documents and Settings\Shawn\Desktop\Server\sendmail\sendmail.exe -t"
; Force the addition of the specified parameters to be passed as extra parameters
; to the sendmail binary. These parameters will always replace the value of
; the 5th parameter to mail(), even in safe mode.
;mail.force_extra_parameters =
Of course these are my settings I have set up, you would need to replace localhose with your server and change the path to your sendmail executable.