nnmail-crosspost-link-function is a variable defined in `nnmail.el'.
Its value is
add-name-to-file
This variable may be risky if used as a file-local variable.
Documentation:
*Function called to create a copy of a file.
This is `add-name-to-file' by default, which means that crossposts
will use hard links. If your file system doesn't allow hard
links, you could set this variable to `copy-file' instead.
You can customize this variable.