BuddyPress.org

ok, here’s the results:

1. created a full mirror of my live site in a test directory, including a separate copy of the database.

2. de-activated all plugins, including buddypress

3. chose a non-buddypress theme in admin

4. ftp’d a fresh copy of wpmu to the directory – surprisingly, this didn’t break anything.

5. Before re-activating any plugins, I successfully registered, all activation / notification emails worked fine.

6. re-activated BuddyPress

7. tried to register a new member, and now I have nothing – no activation / notification emails.

Am I wrong to conclude this is a BuddyPress issue?