However, little differnces between implementations can be annoying. When my ISP changed its mail server software, all my folders suddenly became sub-folders of INBOX, which meant that I had to download mail for all the subfolders again.
And now that I've moved from Tiger to Leopard, I find that there are slight differences between versions in the way MacOS Mail handles folders. It complained that I didn't have the INBOX prefix in my prefs as specified by the server. It should have been a simple change, but it then seemed to think that all the folders were different, so it had to go and re-download all my mail.
Downloading 250Mb of data is normally not something that takes hours, but individually requesting each of 4000+ messages takes a lot longer.
And then I have to upload all that email to my backup space on rsync.net - twice, in fact, because I have a separate weekly backup and I'm too lazy to set it up to make use of the files from the daily backup.
Hopefully it will complete by the time I go home. Which is likely, as I have a lot to finish. :-(