nnmail-expiry-target

nnmail-expiry-target is a variable defined in `nnmail.el'.
Its value is

delete

Documentation:
*Variable that says where expired messages should end up.
The default value is `delete' (which says to delete the messages),
but it can also be a string or a function. If it is a string, expired
messages end up in that group. If it is a function, the function is
called in a buffer narrowed to the message in question. The function
receives one argument, the name of the group the message comes from.
The return value should be `delete' or a group name (a string).

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.