“mutt sending mail in iso-8859-1”
To make mutt use koi8-r encoding edit your muttrc file (usually /etc/Muttrc or ~/.muttrc) and add line set charset="KOI8-R" for reading and line set send_charset="KOI8-R" for writing in koi8-r character encoding. Your terminal program and your editor should be configured for koi8-r encoding independently/separately.