message-indent-citation-function

message-indent-citation-function is a variable defined in `message.el'.
Its value is

message-indent-citation

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

Documentation:
*Function for modifying a citation just inserted in the mail buffer.
This can also be a list of functions. Each function can find the
citation between (point) and (mark t). And each function should leave
point and mark around the citation text as modified.

You can customize this variable.