in reply to Describe your Dream WebMail App
POP/IMAP (or some simple format) export, or forwarding. Most people who receive a significant amount of email have a non-webapp heavily configured to deal with it (e.g. Gnus, VM, mutt, Mail.app, ....), and find webmail a pain. The best thing you can do for these people is to make the webmail easily available through that client.
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