default-sendmail-coding-system

default-sendmail-coding-system is a variable defined in `sendmail.el'.
Its value is

utf-8-unix

Documentation:
Default coding system for encoding the outgoing mail.
This variable is used only when `sendmail-coding-system' is nil.

This variable is set/changed by the command `set-language-environment'.
User should not set this variable manually,
instead use `sendmail-coding-system' to get a constant encoding
of outgoing mails regardless of the current language environment.
See also the function `select-message-coding-system'.