Hi wolfram,
NUKI definitely can do this technically. I am not familiar with IFTT, so I can not garantier you that you can do it there or you have to program it yourself. You probably need the bridge as well.
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…
I did this in the past with Gmail filters/labels, Google Apps Script and IFTTT. When Google Apps script would find an unread mail with a specific label, it would call IFTTT to open the door with Nuki. Performance was not the fastest, but acceptable (30 seconds to 2 minutes).