My users didn't have a problem with this, but good idea!

I think my participants had no porblem with that, but probably only because they didn't think about it since it wasn't a real situationand they didn't really needed to know if each contact would reed the mail. I could imagine I wouldn't be sure how it works and how/if I get a feddback for all recipients (bcc/cc) or just one

Yes, I can imagine that you would need feedback from only the main recipient, for example.

What is the use case here? In what situation do I want to be sure that some recipients received my mail, but I don't care that much that others did?

Maybe the problem is rather that you would want to know if everybody gets the e-mail but there is no clear feedback if choosing "delivery status" means you will get the delivery information for everybody or just for the "main"recipient

Let's see if others had this issue as well.

I think it is important that when you receive the recipient feedback, you can see who received the mail and who didn't – which should be the case with the current function, right? But maybe some people thought you couldn't see that or just were not sure. So if you once get the "real" recipient feedback, showing who really received the mail, the problem might be solved. So more a problem of the use test situation/simulation, than of the real use.
...do you understand what I mean?

In T3616#53827, @Nikolai wrote:

I think it is important that when you receive the recipient feedback, you can see who received the mail and who didn't – which should be the case with the current function, right? But maybe some people thought you couldn't see that or just were not sure. So if you once get the "real" recipient feedback, showing who really received the mail, the problem might be solved. So more a problem of the use test situation/simulation, than of the real use.

Indeed