pre-define or alter list of IMAP keywords

[spi]

I am using Dovecot 2.3.4.1 with sdbox and mostly Thunderbird on Linux
and FairEmail on Android as IMAP clients.

I use a defined set of IMAP keywords to allow different users to mark
and filter their mails based on keywords. That works pretty well but I
stumble across an issue from time to time:

In TB I can pre-define a list of keywords which the user can choose one
from.

FairEmail shows all those keywords Dovecot presents - there is nothing
like the list in TB.

Issue 1: If there are no mails flagged in a folder, Dovecot doesn't
present any keywords and therefore FairEmail shows none - the user has
to remember and type in the correct keyword.

Issue 2: After some time FairEmail shows a huge list of keywords for the
INBOX, even keywords not in use for quite some time. To get rid of these
keywords I deleted the index files on Dovecot (which with sdbox are not
meant to be deleted?). It seems, Dovecot caches the keywords in the
index files.


Question:

1) Is there a way on Dovecot to define a list of keywords Dovecot
presents to the IMAP clients?

2) If not, is there a nicer way to clean the keyword lists from old
entries not assigned to any mail in that folder?

--
Cheers
spi
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