What is the email client you are using and what is the mobile platform? It has been a long time since I worked with deeplinks but I think the generally preferred method for deeplinking is to use a service for it. The reason is the deeplink method is different between mobile platforms (iOS, Android) and Web (which you need to account for since the email can just as easily be opened on a desktop).

If you're just doing some basic testing, I recommend you make sure the deeplink protocol you're using is the correct one for your mobile platform. If this is going to be a production feature, take a look at either (1) implementing your own deeplink service or (2) using a pre-built deeplink service.