result, you don't get a warning if you try to send the same message
twice.
-@vindex sendmail-coding-system
+@c This is indexed in mule.texi, node "Recognize Coding".
+@c @vindex sendmail-coding-system
When you send a message that contains non-ASCII characters, they need
to be encoded with a coding system (@pxref{Coding Systems}). Usually
the coding system is specified automatically by your chosen language
environment (@pxref{Language Environments}). You can explicitly specify
the coding system for outgoing mail by setting the variable
-@code{sendmail-coding-system}.
+@code{sendmail-coding-system} (@pxref{Recognize Coding}).
If the coding system thus determined does not handle the characters in
a particular message, Emacs asks you to select the coding system to use,