mail-yank-hooks

mail-yank-hooks is a variable defined in `sendmail.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• This variable is obsolete since 19.34;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 t).
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It is obsolete and mail agents should no longer use it.