[CentOS] setuid or other ideas

Everyone,

I am working on a Centos 5.9 system. I have an need to be able to activate a piece of software from /etc/smrsh that is activated when sendmail delivers the e-mail to this piece of software. I would like this piece of software to take on the user and group identities that are different than 'mail' which is what happens now. I want to use a user and group that is not root), so that the piece of software will be able to write (concatenate) to a file.

I have never used setuid, but it appears that this will only allow a piece of software to be set to root. I really do not want to give that kind of privilege to this piece of software.

Any ideas?