[Dovecot] Upload / migrate old mails: iPhone shows oldest mails first

hi,

I've got a very strange behavior: I uploaded old mails from a local folder (thunderbird / osx) to my Dovecot (Debian Squeeze) and now, the iphone (ios6) show the oldest mails first. The only thing I can imagine is, that Dovecot got from Thunderbird the newest mails first and last the old mails, so that in the last mdbox are the old one. So, if it is correct, how can I fix it?

cu denny