[Dovecot] Why SETACL accepts non-existant users (was Re: Why are ACLs for non-existent mailboxes accepted?)

[Boris]

On Friday 21 February 2014 08:54:34 Steffen Kaiser wrote:
> On Thu, 20 Feb 2014, Boris wrote:
> > On Thursday 20 February 2014 20:45:32 Boris wrote:
> >> Dovecot 2.2.9-1 accepts SETACL commands that share mailboxes to
> >> non-existent mailboxes. There is no error message. Is this intended
> >> behavior?
> 
> There is a "false friend" here. A "mailbox" in the tongue of many English
> speakers is an IMAP folder, the mailbox file all mails had been appended
> together in the old times, when mbx or mbox mail storage format was
> common. In Germany many people use "Mailbox" as the collection of all IMAP
> folders of one account.
> 
> So the question is why Dovecot accepts non-existant _users_ as you wrote
> in your last line.
> 
> > I probably found the solution myself. Quoting RFC 4314:
> No, because of mailbox != Mailbox.

So what is the reason then?