Not only can you create custom email signatures for your desktop or phone, you can create them for each different device and email software combination.

For example, if you are on your personal email account while at your desk you can have a specail signature for that. And for the same email account but on your phone, you can have the exact same signature or create a different one that indicates you are on mobile.

But if you've ever been on a trip and your laptop and/or phone is dead, you may be forced to use the hotel's 'business center' to access your email through a browser. Never fear, you can even setup your email signature in the email provider's web interface. So not matter where you are in the world and you use the web interface, you can still have those custom email signatures stored on the email provider's email server.