You need the template token module to insert a deeplink to a specific entity. Make a virtual attribute in your entity that can be read via a token and insert it in the mail. This will look like
https://myapp.mendix.com/link/entity/7283947
Deeplinks are generated in the following format :
http://url/link/deeplinkname/attribute
The attribute is the one that you specified for the related object when you created the DeepLink.
So you can simply use this format in your email and only change the attribute per custom email.
Be sure to configure your login and index pages correctly.