feedmail-message-id-suffix

feedmail-message-id-suffix is a variable defined in `feedmail.el'.
Its value is

nil

Documentation:
If non-nil, used as a suffix for generating unique Message-Id: headers.
The function `feedmail-default-message-id-generator' creates its work based
on a formatted date-time string, a random number, and a domain-looking suffix.
You can control the suffix used by assigning a string value to this variable.
If you don't supply one, the value of the variable `user-mail-address' will be
used. If the value of `feedmail-message-id-suffix' contains an "@" character,
the string will be used verbatim, else an "@" character will be prepended
automatically.

You can customize this variable.