If your system doesn't have sendmail, it's probably got some other MTA which could possibly emulate sendmail's "-t" option.

If you are wanting to send mail to another SMTP server aside from localhost, you are going to have to talk the SMTP protocol in your script, or write a script to act like sendmail -t, and pass the message off to it.

The best solution really depends on your situation; what platform are you dealing with, and what are your choices for sending mail locally? Sending through a remote SMTP server would be my last choice.