mail-source-delete-incoming

mail-source-delete-incoming is a variable defined in `mail-source.el'.
Its value is

10

Documentation:
If non-nil, delete incoming files after handling.
If t, delete immediately, if nil, never delete. If a positive number, delete
files older than number of days.

Removing of old files happens in `mail-source-callback', i.e. no
old incoming files will be deleted unless you receive new mail.
You may also set this variable to nil and call
`mail-source-delete-old-incoming' interactively.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.2 of Emacs.