[Dovecot] IMAP message ID bad for very old message

[a.smith at ukgrid.net]

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????

thanks Andy.