If you read mail with Rmail, set the variable mail-yank-prefix.
For Gnus, set message-yank-prefix. For VM, set
vm-included-text-prefix. For mh-e, set mh-ins-buf-prefix.
For fancier control of citations, use Supercite (see the Supercite Manual in The Supercite Manual).
To prevent Emacs from including various headers of the replied-to
message, set the value of mail-yank-ignored-headers to an
appropriate regexp.