Sendmail - Any Ideas?

I have solaris 9 and am using sendmail to pickup requests and forward them to a bulk mail server on a different port. Now for the fun...In sendmail, I need to find a way to place a default address in the Mail From: and Rcpt To: or remove them as required - These will be picked up by the bulk mail sender and sent out. (Sendmail is in between to act as a buffer to a flexible SMTP Connection) Any ideas ? Thanks!

Have the original email received by a process (set up as an alias "|myprocess") and have this process modify the contents of the email and then pass the modified file to the bulk mail server.

Why do you want to do this? Do you have a problem with spam filters?

The upstream application does not have the ability to recover when the SMTP Connection (eventlistener)goes down on my bulk mail server. If I can place Sendmail in between, when the eventlistener is down, the mail will queue up in Sendmail. (By design, my bulk email server eventlistener will recycle itself to recover from errors).
I have this - Upstream Application ------> SMTP--->EventListener (Mail Server)
Would like to do this - Upstream Application ----->SMTP---->Sendmail---->SMTP--->EventListener(Mail Server) - Thanks!