Within the Mail.app preferences for "Accounts" is a section called
"Mailbox Behavior" where you can set things to automatically delete.
If the box is not checked, then things never go to this trash but are
immediately deleted. If it is set to automatically empty, they are
deleted from trash immediately.
Only trash things that you never want to see again. If you want a "pre-
trash" folder of some type, create one separately. All of the
electrons that were mysteriously placed into the trash folder before
are picked up each night by the Physics Fairy and recycled for new
email messages, in keeping with various magical laws related to the
conservation of matter and energy and observed by all hippies from
Cupertino.