[Dovecot] Unique message IDs?

[Alexander Chekalin]

>>> With Maildir the message GUID is typically the same as the Maildir base filename (i.e. everything before ':' character).
>>
>> But what if I one day decide to convert my maildir's to mbox'es?
>> I really plan to do such conversion in a while (as soon as I finish the
>> indexing system).
>
> mbox? or mdbox? With mbox there are no proper GUIDs, but Dovecot kind of
> fakes it by returning MD5 of specific headers as GUIDs (so not 100%
> reliable). With mdbox GUIDs work even better than with Maildir, the GUID
> is always stored in the message's metadata.

I'd like to use the best optimized one (mdbox), but there is a reason 
not to do that is when I use mbox or maildir I can see where given IMAP 
folder mails are stored, so, say, if I want to copy only one IMAP folder 
to some remote site, I just copy know dir or file. With mdbox this is 
different, I simply can not guess where my messages exactly are.

If it be possible to have per-IMAP-folders mdboxes, I love to use it. 
But from what I know and tried this is not way mdbox used to work, right?

Yours,
   Alexander Chekalin