ImapGoose

Summary

ImapGoose is a small program to keep local mailboxes in sync with an IMAP server. The wording “keep […] in sync” implies that it does so continuously, rather than a one-time sync. ImapGoose is designed as a daemon, monitoring both the IMAP server and the local filesystem, and immediately synchronising changes. When the IMAP server receives an email, it shows up in the filesystem within a second. When an email is deleted on another email client, it is removed from the filesystem within a second.ImapGoose is highly optimised to reduce the amount of network traffic and tasks performed. To do so, it relies on a few modern IMAP extensions and only supports modern email servers. “Modern servers” in the context of email means servers which support extensions which were standardised between 2005 and 2009.ImapGoose uses the CONDSTORE extension (standardised in 2006), which basically allows it to tell the server “I last saw this mailbox when it was in state XYZ, please tell me what’s new”. This avoids the need to download an entire message list (which can be tens of thousands of emails), making incremental syncs much more efficient. It also uses the QRESYNC extension (standardised in 2008) so that the server includes a list of deleted messages too (i.e. VANISHED). Finally, ImapGoose uses the NOTIFY extension (standardised in 2009), which allows an IMAP client to tell the server “please let me know when there are changes to these mailboxes”, and then leave a connection open. NOTIFY has two nice consequences: (1) the client doesn’t need to ask the server if there have been any changes at regular intervals, and (2) the client is informed of any changes immediately, so they can be processed without delay. Unlike the older IDLE extension (from 1996), NOTIFY (from 2009) allows monitoring multiple mailboxes per connection, rather than just one.In this article, I’ll cover some of the general design details, inner workings and other development details.General mode of operationFirst o...