It is a table in the ubb database.
For example if your host provides a control panel such as cpanel. You will usually have at least phpmyadmin.
select it.
Then select the ubb database.
You will have around 64 tables depending on your ubb version.
Scroll down to PRIVATE_MESSAGE_POSTS
select it.
Done.
Just fyi, I keep my nose out of it.
I for one would want to think that the pm system is totally private.
But beware you may find somebody talking about you. LOL.
