Well, you have the webapi Nuki Web API. You can trigger an “open” command by any means. The api looks complicated in the beginning, but with some beginner/intermediate coding experience you can write your own program within a couple of hours. For me the most difficult part would be to implement the processing of the email. If this can be handled by iftt, it should be easy to implement.
Any particular reason to use emails? They are slow and this is very annoying when you are waiting in front of a door. There should be lots of alternatives…