This is a great safety net, but eventually you should empty that trash to recover the disk space used by them.
Empty the Deleted Messages Folder Quickly in Mac OS X Mail
A fast way to empty the Deleted Messages folder in Mac OS X Mail is via the keyboard:
- Press Command-Shift-Backspace.
- In Mail 1, 2 and 3, press Command-K.
- Click Erase or OK.