Actually, maybe not. I found a wiki page which includes a section on using dovecot as a local-to-your-N900 intermediary IMAP server to protect Modest from the vagaries of your internet connection.

Let's face it: Modest really isn't up to the challenge of being a direct IMAP client for e.g. a busy GMail Inbox, (even when making use of categories to limit what IMAP discloses).

I haven't tried this yet, but I think I'll give it a go over the next few days when I have time to mess with it.