please let me know by return?

[Tedwonny]

I am writing an email and was wondering if the following sentence is correct. Thanks a lot in advance

1) Please let me know by return. (e.g. I want to know the receiver's answer)

Also, I'd like to know if the following sentence is correctly written.

I’ve checked each case against the students’ documents which you have copied me.

[SoothingDave]

No. "Return" is not used like that.

More to the point, if you send someone email and ask them for something, you naturally expect them to email you back.

"Please let me know." should be sufficient.

If you want to specifically ask them to use email and not call you on the phone, or send a letter or fax, then say "Please respond by email."