It was a notification of a private message. So the notification came from Tascam Forums, but the private message was sent by a spambot.
I did delete almost all users that registered last Friday and Saturday (unless they were obviously humans), so Jarno is right--the PMs probably disappeared when the users were deleted.
I did delete almost all users that registered last Friday and Saturday (unless they were obviously humans), so Jarno is right--the PMs probably disappeared when the users were deleted.