IMAP search with non-ASCII characters fails

[GLingDroid]

Expected behavior

It should return emails that contain the keyword in the mail title.

Actual behavior

It say 'search fails' and return nothing which the one exists on the server.

Steps to reproduce

1.check enable 'server-side search' in the account settings.
2.search like using chinese character '你好'.
3.hint the button of search on server.
4.it tells 'search fails'.

But it's ok when you use english character like 'hello', it performs as indeed.
I think it's related to character code(ISO... ), like utf-8,GB18030,utf-16...

Environment

K-9 Mail version:5.600

Android version:7.1.2

Account type (IMAP, POP3, WebDAV/Exchange):IMAP,gmail

[cketti]

We need to use the CHARSET argument when using IMAP's SEARCH command. See https://tools.ietf.org/html/rfc3501#section-6.4.4

[cketti]

Please note that there's no need to provide specific examples where search also fails. This problem happens when any non-ASCII character is used.