If your mobile phone email address isn't in the list provided in the back office, or on this list, it means you won't be able to receive text message notifications on lead activity. 

A mobile phone email address simply converts an email into a text message format you can easily receive on your mobile device. By inserting a mobile phone email address in the proper format in this slot, you're just telling the system to convert your email notification into a text notification as well. Not all mobile carriers provide these to users. If you want to be extra sure your phone carrier doesn't offer this, you can call them and ask them about it. 

A mobile phone email address has absolutely no bearing on your ability to send out text message autoresponders or broadcasts from the system, as this feature sends out text messages using a number from the system. The only thing it affects is your ability to receive text notifications on lead activity.