If doveadm says it's invalid UTF-8, it's invalid UTF-8. I guess your terminal isn't actually using UTF-8 then, but something else. ("locale" output should say something about UTF-8.) I guess doveadm could also automatically translate parameters to UTF-8, but that's a bit annoying to implement.
You were absolutely right. After thorough testing I could make up the problem to the SSH-Client PuTTY I was using on a Windows machine while testing. The default "remote character setting" is "ISO-8859-1:1998 (Latin-1, West Europe)", when re-setting to "UTF-8", opening a new shell and testing the "doveadm mailbox rename ..." with german umlauts just works fine then. (Just for the sake of completeness, the "locale" settings were set to (LANG=de_DE.utf-8) globally in /etc/sysconfig/i18n per default.)
The problem here is that * is expanded by your shell, not doveadm. And it expands into Tr&-AOQ-sh as it's in the filesystem, but that's only the mUTF-7 encoding of it. The UTF-8 version of the name is Tr&AOQ-sh. So doveadm only sees that you attempted to resync a nonexistent mailbox. Using '*' with quotes would work, since doveadm would do the expansion then.
Thanks for pointing that * / '*' issue out.
I now unterstand that the "doevadm mailbox rename" converts the input to UTF _before_ applying it in the filesystem.
Now it makes sense that doveadm mailbox rename -u user 'Trash' 'Tr&AOQ-sh' must be expanded to Tr&-AOQ-sh. The "-" character directly after the "&"in Tr&-AOQ-sh comes from a special mUTF-Specification (as stated in RFC 3501, section 5.1.3):
"In modified UTF-7, printable US-ASCII characters, except for "&", represent themselves; that is, characters with octet values 0x20-0x25 and 0x27-0x7e. The character "&" (0x26) is represented by the two-octet sequence "&-"."
So e.g. if I wanted a german umlaut to be encoded in the filesystem, I must enter it directly into dovedm instead of the UTF encoded value.
One small point left...
The UTF-8 version of the name is Tr&AOQ-sh Just for understanding - "Tr&AOQ-sh" is IMHO UTF-7, not UTF-8. Accordingly to what stated before, "Tr&-AOQ-sh" and "Tr&AOQ-sh" are encoded both the same (UTF-7), the first seen in clients as "Tr&AOQ-sh" and the second as "Träsh".
Thanks for all your help! Megodin
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