We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/checking-for-new-mail/261
Mailspring continuously syncs with the mail server, so messages appear as soon as they’re available on the server. Mailspring will automatically reconnect and sync after being disconnected.
If you lose your connection, Mailspring will display an alert in the bottom left of the main window.
To force Mailspring to re-sync all accounts immediately, choose "Sync New Mail Now" from the application menu.