mail-send-nonascii

mail-send-nonascii is a variable defined in `sendmail.el'.
Its value is

mime

Documentation:
Specify whether to allow sending non-ASCII characters in mail.
If t, that means do allow it. nil means don't allow it.
`query' means ask the user each time.
`mime' means add an appropriate MIME header if none already present.
The default is `mime'.
Including non-ASCII characters in a mail message can be problematical
for the recipient, who may not know how to decode them properly.

You can customize this variable.