Just FYI, UBB.threads was never meant to be used for a newsletter system (it was, and will remain, a Forum software title); the internals are there to send out mailings, but I'm sure you'll just end up getting your IP blacklisted (trust me, I've been there, plenty of times).
Generally we'd recommend sending messages out in groups so that you're not blasting 10k people at once (which may result in some email providers blocking you from their systems). Plus, there will be bounces (I actually made a script to deal with email bounces at some point).
Generally email campaigns are sent from private services such as
Sendy (I have not used this service, it was recommended to me by a colleague), and they do all of the footwork of getting their IP's removed from blacklists, handling bouncebacks, etc.
