Turn off all possible message changes for some users?

[Furnish, Trever G]

Seems like a straightforward question but I just want to make sure I'm not
missing something before I deploy.

For our next phase of testing I want to have mailscanner start handling all
the mail for several of our high-volume, many-user domains, but I only want
a small subset of those users to actually be part of the testing - email for
everyone else ought to be untouched (at least as far as a user is likely to
notice).  What is the proper / best way of doing that?

So far I've turned "Virus Scanning" and "Spam Checks" into rulesets.  I've
also configured the MTA (sendmail) to split messages with multiple
recipients into one message per recipient.  The rulesets look like so:

To:     testuser at testdomain.com yes
FromOrTo:       default no

I also set "Scanned Modify Subject" to a similar ruleset.

Anything else I'm missing and should turn off for non-test users before I
deploy?

--
Trever