I also opened a thread on this on Drupal.org but since it's been awhile I thought I'd try asking here.
Use Case: In general, I want users to receive notifications for new private messages. I can do this using the included submodule for e-mailing private messages.
So that the private message inbox has a message when users log in for the first time, I used the Rules module to automatically send a "welcome" private message upon registration on behalf of an administrator account. However, this triggers the "send e-mail on private message" behavior. Since users already are receiving a "please validate your e-mail address" e-mail, I don't want them to receive a second e-mail notifying them that they have received the "Welcome" private message.
Is there a way to prevent messages from certain users (or, alternately, certain messages) from triggering the e-mail notification?
I also looked into implementing this using rules instead of the included submodule but this would mean I couldn't use the functionality for reducing the number of notifications (currently, I have it set to not send additional e-mails if the user has yet to log in and read the first e-mail in a thread from a given user).