You can't really. Nowadays most mail servers are locked down to not allow routing from external hosts. This is to help stop spammers that use unprotected email servers as gateways hiding their real location. Even if you were to have each user's admin set their mail server to allow you to use it as a relay, you still have the problem of generating the email on your system and smarthosting to the other systems, so no "load" is averted. The only option you would have would be to queue up the mail data and send it off to their site and have them then queue it -- which seems like a fix worse then the original problem. I would suggest you take a look at setting up queues in sendmail and optimizing your server to handle large queues. Also take a close look at what you are trying to do and make sure you do not have ways that this system can be hijacked by spammers to send anonymous email in batch.

-Waswas

Also note that in no place in this responce does perl get brought up. Your question would be better targeted on a sendmail mailing list.