nnmail-fancy-expiry-targets

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

nil

Documentation:
Determine expiry target based on articles using fancy techniques.

This is a list of ("HEADER" "REGEXP" "TARGET") entries. If
`nnmail-expiry-target' is set to the function
`nnmail-fancy-expiry-target' and HEADER of the article matches REGEXP,
the message will be expired to a group determined by invoking
`format-time-string' with TARGET used as the format string and the
time extracted from the articles' Date header (if missing the current
time is used).

In the special cases that HEADER is the symbol `to-from', the regexp
will try to match against both the From and the To header.

Example:

(setq nnmail-fancy-expiry-targets
'((to-from "boss" "nnfolder:Work")
("Subject" "IMPORTANT" "nnfolder:IMPORTANT.%Y.%b")
("from" ".*" "nnfolder:Archive-%Y")))

In this case, articles containing the string "boss" in the To or the
From header will be expired to the group "nnfolder:Work";
articles containing the string "IMPORTANT" in the Subject header will
be expired to the group "nnfolder:IMPORTANT.YYYY.MMM"; and
everything else will be expired to "nnfolder:Archive-YYYY".

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.