Mailspring keeps a connection open to each account and shows new messages as soon as the server has them; you don't need to refresh. If you want to force a check, choose Sync New Mail Now from the main menu.

"Syncing your mailbox..."

The spinner in the bottom-left of the sidebar means the sync engine is still working through your mailbox. Click it to see each folder's progress: Scanning is the message list, Caching mail is message bodies. For a new account with a large mailbox this can take hours — recent mail arrives first and you can use Mailspring normally while older mail fills in. See Why don't I see all my old mail?.

"One or more accounts are having connection issues"

This banner appears whenever any account's connection fails, even briefly (for example after your computer wakes from sleep). It usually clears on its own or when you click Try now. If it keeps coming back, go to Preferences > Accounts: the affected account is shown in red with the reason. Click Reconnect to sign in again (this is the fix when a password changed or an OAuth token expired), or Error Details... to see the recent sync log.

If sync seems stuck

1. Make sure you're on the latest version of Mailspring.
2. Select the account in Preferences > Accounts and click Rebuild Cache.... Mailspring re-downloads that account from the server.
3. If every account is affected, use Preferences > General > Reset Cache instead, which rebuilds the whole local database. Your accounts and settings are kept.
4. On Windows, add Mailspring's data folder to your antivirus's exclusions. Real-time scanning of the database file is a common cause of stalled syncs.