When you add an account, Mailspring doesn't download everything at once. It fetches your newest messages first so you can start working within a minute or two, then keeps walking backwards through each folder in the background. For a mailbox with tens of thousands of messages, reaching the oldest mail can take hours — sometimes a day or more — and it only progresses while Mailspring is running.

Checking progress

While the sync is running, Syncing your mailbox... appears at the bottom of the sidebar. Click it to expand a per-folder list: Scanning (42%) means the message list is still being downloaded, Caching mail (80%) means bodies are being fetched. Once a folder shows no progress label, its full history is in the list.

What's kept on your computer

• The message list (sender, subject, date, read state) for every message in every folder, once the initial sync finishes.
• Message bodies and attachments for mail from roughly the last three months, plus any older message you open. Opening an old message downloads it on the spot; Mailspring drops old bodies from the cache again a couple of weeks after you last fetched them. Spam and Trash bodies aren't cached.

This is why search finds older mail by sender or subject but may not find a phrase from the body of a two-year-old message, and why very old messages take a moment to open when you're offline. There isn't a setting to change the window or to cache an entire mailbox offline; if you need a full local archive, right-click a folder and use Export folder as .mbox file....

If it stops making progress

Give a large mailbox at least a day. If a folder's percentage hasn't moved in that time, select the account in Preferences > Accounts and click Rebuild Cache.... On Windows, exclude Mailspring's data folder from antivirus real-time scanning. If a specific folder never syncs, check whether your provider hides it from IMAP (Gmail's Show in IMAP setting is the usual culprit).