I was so tired of getting the 'memory almost full' icon on the top of my screen. My 'Email' folder had 32mb of data which was all in the Deleted email folder, 2000 emails. Here's how to clear it all out.
Go to 'Settings' icon and hit it. Go to "Applications"
Then hit 'Manage applications'
Do a sort by size. Email will probably be #1. If not, find it. Click on it and you will be in 'Application info'.
You see the Data line. Mine was 32mb.
Hit clear data. Now this will clear everything. You might have to re enter your mail accounts, but that is very easy to do. All that memory is now freed up for internal use!
BTW, d/l from the market aCCleaner. It will be very helpful in cleaning caches and data. In addition get App 2 SD Free. It will move all the apps that can be moved into your SD card. This frees up an incredible amount of memory. You will no longer have to look at the menacing hd icon on the top of the phone telling you you are out of memory.