Собственно, тривиальный .mbsyncrc:

IMAPAccount imap-lockywolf-yandex
Host imap.yandex.ru
User <redacted>
PassCmd "my_get_password"
SSLType IMAPS

IMAPStore imapstore-lockywolf-yandex
Account imap-lockywolf-yandex

MaildirStore local-lockywolf-yandex
SubFolders Verbatim
Path ~/Mail-mbsync/
Inbox ~/Mail-mbsync/inbox

Channel yandex->local
Master :imapstore-lockywolf-yandex:
Slave :local-lockywolf-yandex:
Patterns *

Запускаем mbsync -V -l 'yandex->local'

Видим:

lockywolf@delllaptop:/tmp/RAMFS$ mbsync -lV 'yandex->local'
Reading configuration file /home/lockywolf/.mbsyncrc
Channel yandex->local
Opening master store imapstore-lockywolf-yandex...
Resolving imap.yandex.ru... ok
Connecting to imap.yandex.ru (77.88.21.125:993)...
Opening slave store local-lockywolf-yandex...
Connection is now encrypted
Logging in...
Authenticating with SASL mechanism PLAIN...
INBOX
&BB4EQgQ,BEAEMAQyBDsENQQ9BD0ESwQ1-
&BBgEQQRFBD4ENARPBEkEOAQ1-
&BCEEPwQwBDw-
&BCMENAQwBDsENQQ9BD0ESwQ1-
&BCcENQRABD0EPgQyBDgEOgQ4-
Archive

Ну ништяк, в 2020 году проблемы с кодировкой. Как поправить?

FS – ext4, системная кодировка utf8.

iconv -l показывает миллион всяких разных кодировок.