message-courtesy-message

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

"The following message is a courtesy copy of an article\nthat has been posted to %s as well.\n\n"

Documentation:
*This is inserted at the start of a mailed copy of a posted message.
If the string contains the format spec "%s", the Newsgroups
the article has been posted to will be inserted there.
If this variable is nil, no such courtesy message will be added.

You can customize this variable.