feedmail-queue-default-file-slug

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

t

Documentation:
Indicates what to use for subject-less messages when forming a file name.
When feedmail queues a message, it creates a unique file name. By default,
the file name is based in part on the subject of the message being queued.
If there is no subject, consult this variable. See documentation for the
function `feedmail-queue-subject-slug-maker'.

If t, an innocuous default is used.

If a string, it is used directly.

If a function, it is called with no arguments from the buffer containing the raw
text of the message. It must return a string (which may be empty).

If the symbol 'ask, you will be prompted for a string in the mini-buffer.
Filename completion is available so that you can inspect what's already been
used, but feedmail will do further manipulation on the string you return, so
it's not expected to be a complete filename.

You can customize this variable.