mailclient-place-body-on-clipboard-flag

mailclient-place-body-on-clipboard-flag is a variable defined in `mailclient.el'.
Its value is

nil

Documentation:
If non-nil, put the e-mail body on the clipboard in mailclient.
This is useful on systems where only short mailto:// URLs are
supported. Defaults to non-nil on Windows, nil otherwise.

You can customize this variable.