I don't have the Strat, so I'm just throwing out ideas here, but try long pressing one of the emails and see if it gives you an option to delete multiple emails. Another place to try is pressing menu while in your inbox and see what options are in there. Also, there may be an option to choose how many days worth of emails your phone syncs with. After this amount of time, they should start dropping off your phone.
YES YOU CAN!
You can actually "Delete Converstaions"
Go to Email Menu - More - View Mode and set your View to "Conversations"
Each "conversation" is shown by a "drop-down" arrow to the left of the Conversation "Subject".
Press and Hold the "Subject Text" (do not press the drop-down arrow).
A menu will Pop-Up and "Delete Conversation" is an Option.
Then go empty your trash!
When I go to the trash folder on my Stratosphere, I can check individual emails and then hit "Menu" and "Delete", but if I hit the "Menu" button before selecting any e-mails, then I see the "Delete all" button, but it's greyed out, so I cannot select it. There's also a "Restore All" button next to the "Delete all" button, and it's also greyed out. So, currently the only way I can delete the 100's of e-mails in my trash is to put a check mark next to each one, which is painful. Anybody else have this problem or know of a solution?
My son went to apps. and got me an app. called k-9 mail and it deletes like 20 at a time. Alot faster than one at a time, espically when i had like 5,682 emails that had to be deleted. I don't know what you have to do when you get the app. but he did it pretty fast, you know these kids are so smart when it comes to these things. Well hope this helped.