message-indentation-spaces

message-indentation-spaces is a variable defined in `message.el'.
Its value is

3

Documentation:
*Number of spaces to insert at the beginning of each cited line.
Used by `message-yank-original' via `message-yank-cite'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.