[Dovecot] Encoding dovecot keywords

Hello.

If mailbox name include non latin chars, dovecot save it encode to modification UTF-7. Example "тест" convert to "&BEIENQQ6BEEEQg-". If replace & to + I can decode it:

<?php $str = '+BEIENQQ6BEEEQg-'; echo iconv('UTF-7', 'UTF-8', $str); ?>

But I can't understand what encoding use dovecot for keywords? I set "тест" (number 5) keyword:

cat dovecot-keywords 0 &bcienqrbbeiepgqybdaetw-_&bdwenqrcbdoema- 1 test 2 $label1 3 $label2 4 $label3 5 &beienqq6beeeqg-

but "&beienqq6beeeqg-" do not convert to "тест". Why?