Creating folders with creepy names if default "sent" folder has cyrillic name on server

I tried creating a mailbox named “Отпаравленые” and then move a message to it. This worked without problems (with a Dovecot IMAP server).

Non-ASCII mailbox names have to be encoded/decoded by MailMate and the server. I believe MailMate follow the standards, but I'm of course willing to look into it if Yandex appears to behave correctly. Does it work with other email apps?

Where do you see the badly encoded name? In MailMate or elsewhere?

Is it just this mailbox name or is it any non-ASCII mailbox name which misbehaves?

Is there any way I can obtain a test account?

Non-ASCII mailbox names have to be encoded/decoded by MailMate and the server. I believe MailMate follow the standards, but I'm of course willing to look into it if Yandex appears to behave correctly. Does it work with other email apps?

Thuderbird doesn't have this issue.

Where do you see the badly encoded name? In MailMate or elsewhere?

In Thunderbird, MailMate and web interface of yandex mail

Is it just this mailbox name or is it any non-ASCII mailbox name which misbehaves?

All other folders work well
I effectively have Отпаравленые folder twice, and MailMate sees both, but uses the badly-encoded one to store sent emails.

Is there any way I can obtain a test account?

I've tried to create a fresh account, but for some reason bug doesn't trigger. I'll experiment around what could have caused it.

Could it be due to fact that IMAP account was imported from Thunderbird, instead of being created from scratch?

Yes, I think it could be the import. Try right-clicking the correctly named mailbox and select “Sent Messages” for “Mailbox Type”. Does that clear up the problem?

If it does then the problem is that MailMate does not import non-ASCII mailbox names from Thunderbird “correctly” (there is no standard way to do such an import, it's based on trial-and-error).

I've experimented around it and found this:

This seems to only happen with account imported from thunderbird. And only before this "Отпаравленые" folder starts to sync for the first time. After this "Отпаравленые" folder starts downloading its own messages, newly sent email will go to correct "Отпаравленые" folder instead of &BCcENQRABD0EPgQyBDgEOgQ4- one.

I have around 10 gig of email conversation, so it takes time to download all other folders, I guess this is the main reason why it didn't trigger for small test account. Still, I'll send you login data for yandex mail I've created, so that you'll have it in case you'll need it for future experiments(see mm-feedback email for credentials)

Was the small test account also imported from Thunderbird?

I wasn't able to trigger it with test account: I need to have internet in order to send an email, but it seems that I need to have "sent" folder not synced yet.
I was able to trigger same bug by reimporting my main account from Thunderbird, but I can't trigger it when starting from scratch.