I would like to create an email client that can access multiple IMAP mailboxes. I'd also like a copy of all emails for processing. What is the best way to do this using IMAP commands?
Right now I have a script that iterates over the folders, FETCHing FLAGS on 1:* to see what's been read and if any previously read messages have been marked as new, then FETCH BODY.PEEK on all of the messages I don't have in my database. Is there a better way?
A better way would be to fetch UIDs of all messages (UID FETCH 1:* FLAGS), compare the resulting UID list with your database and then download any messages you don't have and remove any messages you have but the server doesn't (deleted by other IMAP clients or using a web interface, for example). This is the only reliable method to duplicate an IMAP folder, AFAIK. (And don't forget to take UIDVALIDITY into account as well!)
Your original method would not work correctly if other IMAP clients were accessing the mailbox in addition to your application. In theory, it would work OK if you can stay connected to the IMAP server continuously, using NOOP and IDLE to check new and deleted messages, but this is never possible in practice - even GMail doesn't have 100% uptime :-)
An ultimate IMAP client would combine both these approaches.