OfflineIMAP is a software to dispose your e-mail mailbox(es) as a local Maildir. OfflineIMAP will synchronize both sides via IMAP. The main downside about IMAP is that you have to trust your email provider to not lose your mails. This is not something impossible while not very common. With OfflineIMAP, you can download your Mailboxes and make your own backups of the Maildir. This allows reading your email while offline without the need for the mail reader (MUA) to support IMAP disconnected operations. Need an attachment from a message without internet connection? It's fine, the message is still there.
WWW: http://www.offlineimap.org/
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