The ecommerce package has a solution as part of the customer-service section[1]. It really needs to be separated out into a kind of customer-management-services package.
Also, someone mentioned they are working on an intra-site webmail service, where notifications are tracked and viewed between users within OpenACS without using standard email/sendmail services. WebCT has something like this for user-user and forum-user posts. iirc, the RFC was around the time the contacts package was being discussed --can't find a reference to it right now.
1. "Sending Email to Customers" in https://openacs.org/doc/openacs-5-1/ecommerce/ecommerce-customer-service
