nnmail-scan-directory-mail-source-once

nnmail-scan-directory-mail-source-once is a variable defined in `nnmail.el'.
Its value is

nil

Documentation:
*If non-nil, scan all incoming procmail sorted mails once.
It scans low-level sorted spools even when not required.

You can customize this variable.

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