How Offline Gmail Decides Which Messages To Download

When you enable Offline Gmail, the new service doesn’t actually download all your messages—just about 10,000 of them. And Gmail has its own method of determining which messages it stores for serious email fiends.

Todd Jackson, Gmail’s product manager, gave CNET’s Webware that 10,000 email figure. And Googlers who work on Gmail posted the same 10K estimate, while offering an explanation of their “heuristic” algorithm that tries “not to download uninteresting conversations.”

The short version: Offline Gmail estimates a time span that covers about 10,000 messages and almost all of that period. It definitely syncs any label that’s not too huge, was recently updated, and has some history, including the nearly-guaranteed Starred and Drafts sections. It skips un-labelled Trash and all the Spam, and any label that’s mostly unread.

Here’s the longer, official post on the syncing method:

  • Synchronization is based on the date of conversations. The system estimates a period of time to cover (at least 1 week in length) that results in approximately 10,000 messages being downloaded. For an average user, this means Gmail will end up downloading several years of mail.
  • Additionally, we’ll download any conversation marked with a label that contains less than 200 conversations, has at least one conversation that has been received in the last 30 days and also has at least one conversation that’s outside the estimated time period. For many users, this list of labels will include Starred and Drafts.
  • Finally, the system determines a list of labels to exclude conversations from being downloaded. For example, Trash and Spam are always in this list, along with any label that contains mostly unread conversations (unread count greater than 99%). So, we won’t download a conversation if it contains only labels in this list. A typical Trashed message will not be downloaded, but a Trashed message that contains the label “alpha” will.

Knowing that might give some of us reason to clean out our Gmail accounts and start emptying our inboxes with better labelling systems. Would having only 10,000 messages backed up locally cover only a portion of your inbox? How does Gmail’s choosing methods match up with your email use? Tell us your take in the comments.

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