• the App does NOT need any active connection and can basically be used any time, even while completely offline (no internet connection, no voice coverage, no roaming) and/or while in Flight Mode
• SMS will always work as long as you find yourself in an area with AT LEAST voice coverage on your mobile
• both require you to input a 6-digit One Time Code (OTC) on the device you are accessing your email from

Alternatively, you can be called on your office phone or different phone number (both fixed or mobile), in which case you have to listen to the voice call and hit the hash/pound key (#) when asked to: very useful if you forgot your mobile!