@schrully do you use external plugin and send emails from remote SMTP server?
I also have this problem on my multisite. Maybe it occurs because bbPress send emails with BCC (one email for all subscribed users and mail services like gmail delete emails thinking that it’s spam)?
I remove default functions that send emails and write custom functions to send email notifications with cron and all works correctly right now.
I don’t digg into this issue with default functions. I think the last version of bbPress have bug because previous release sends emails on my site 🙂
