[Dovecot] IMAP message ID bad for very old message

[Timo Sirainen]

On Fri, 2011-05-13 at 09:35 +0100, a.smith at ukgrid.net wrote:
> Quoting a.smith at ukgrid.net:
> 
> >
> > Ok, I just deleted the dovecot index files etc from the mailbox,  
> > when it recreated them all is now looking normal...
> >
> 
> When I logged into my mail today this problem has come back :S Can  
> anyone suggest a cause or anything else I should do?
> I am correct in thinking that Dovecot normally orders emails by date right????

The way it works is that:

Initially when you don't have dovecot-uidlist file, it assigns UIDs for
all the files. It sorts them by the filename, which typically causes
them to be sorted by their received time.

After this when new files are added, the new files are sorted among
themselves and then given UIDs in that order. The new files are always
appended after existing UIDs, never inserted in the middle.

Have you checked if there are any errors in Dovecot logs? There are some
reasons why Dovecot might give a new UID to an existing mail. dovecot -n
output might also be useful.