mh-yank-hooks

mh-yank-hooks is a variable defined in `mh-letter.el'.
Its value is

nil

• This variable is obsolete since 19.34;
  use `mail-citation-hook' instead.
• This variable may be risky if used as a file-local variable.

Documentation:
Obsolete hook for modifying a citation just inserted in the mail buffer.

Each hook function can find the citation between point and mark.
And each hook function should leave point and mark around the
citation text as modified.

This is a normal hook, misnamed for historical reasons.
It is obsolete and is only used if `mail-citation-hook' is nil.