If you are a former Windows user and miss using the key to refresh, you can actually set this up manually on your Mac: Open System Settings (or System Preferences). Go to Keyboard > Keyboard Shortcuts . Select App Shortcuts and click the + icon. Select "All Applications."
Type "Reload Page" or "Refresh" (it must match the exact menu command name) into the Menu Title box. Press in the Keyboard Shortcut box and click Add.
Press Command (⌘) + Shift + N to "Get New Mail" across all accounts immediately.