Rückmeldung can differ slightly by intention as response or feedback (do you have anything to say about this?), or confirmation or acknowledgement (did you receive this?), but some possibilities:

I'd appreciate a brief response.
I'd appreciate some feedback (on this).
I'd appreciate some sort of confirmation (that you got this).
If you could confirm/acknowledge that you got this, I'd be grateful.
If you could tell me if you receive this, I'd appreciate it.
Let me know if you get this(. Thanks in advance).
Please let me know that you've seen this( e-mail).

You can go much crazier and more colloquial or even slang than these, however.