feedmail-queue-slug-maker

feedmail-queue-slug-maker is a variable defined in `feedmail.el'.
Its value is

feedmail-queue-subject-slug-maker

Documentation:
If non-nil, a function which creates part of the queued file name.
Takes a single argument giving the name of the directory into
which the message will be queued. The returned string should be just
the non-directory filename part, without FQM suffix or uniquifying
sequence numbers. The current buffer holds the raw message. The
default function creates the slug based on the message subject, if
any.

You can customize this variable.