Technically, this email is actually from Airbnb, not the guest. They get the details from the guest and then email it to you - like the game telephone. The same is true when you use the contact via text service on the site. You are texting Airbnb and they are texting the guest. If you want a direct email, you need to ask the guest for it.
From a privacy and security concern, you shouldn't expose your email address directly to the guests. So by working outside the Airbnb email system, you are exposing yourself a bit. Also, if the guest responds via email, it is not captured in the Airbnb system should an issue ever arise. That's why it's very important to use their systems to contact the guests.
Did you know you can set up a standard message within the Airbnb platform and save it to send to each guest? This is a very convenient feature.
