Hey - I have the same question! I just got the Moment and set up my email (Comcast). When I deleted emails they ended up in the Trash. I finally figured out that I needed to delete the emails from the server to not have them reappear in the inbox. However - now I have some "deleted" emails in the Trash which I cannot seem to be able to get rid of!
It would be nice to be able to create subfolders for emails that you DO WANT to save on the phone.

 

Not sure if this will help you or not, but I just discovered that to get them to permanently go away in your trash on the phone, you must delete the trash on your email account and then they will automatically disappear once you go to the trash setting on the phone again. Hope it helps!

 

The problem is that you have to first delete them one by one from the Trash, however they are still there, so now you have to hit Menu, Delete Trash - again - one by one! I try to NEVER delete any emails from the inbox (just delete them from the server, then they disappear from the Inbox and you never have to deal with the Trash) and if there are lots of "deleted" emails in the Trash, the fastest way is to delete all the data from the Email and set it up again. Then all is clean again. Of course this it only practical if your main email stash is on your desktop...
The app should be changed so that you can remove all emails from the Trash in one command! I mean there is a reason you put them in the Trash! I do not want them any more!

 

Oh, and it doesn't help to have the delete from server on the phone - you have to delete them from the Comcast server...

 

Interesting, I have the same problem that many of you are experiencing. You would think that there would be an APP to remove, mass delete, and selective mass delete emails from your mobile device.