i have no experience with buunce mail processing, but as i understand it the intention of the modified sendmail file (on the site where OpenACS is running) is to allow the mangled email addresses. Point (a) should not be an issue, but point (b) certainly (if a user can run an OpenACS instance, but cannot not change the mail configuration, and/or is not allowed to run a mail server).
An option to deactivate automated bounce processing and using a fixed mail-bounce address make sense in such situations.
