nnmail-split-hook

nnmail-split-hook is a variable defined in `nnmail.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Hook called before deciding where to split an article.
The functions in this hook are free to modify the buffer
contents in any way they choose -- the buffer contents are
discarded after running the split process.

You can customize this variable.