“ Optimizing Mail’s Envelope Index is very simple. Follow these simple instructions to make Mail snappy again: * Quit Mail from the menu or using Apple + Q and wait until the black arrow disappears from Mail’s Dock icon. * Open up Terminal using Spotlight via Alt + Space or start it from the Applications/Utilities folder. * Type or copy the following line into the Terminal window: sqlite3 ~/Library/Mail/Envelope\ Index vacuum Finally, press Enter to perform the action and wait until it finishes, this can take anywhere from a few seconds to minutes depending on the amount of mail.