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Do the following sentences mean the same?

1: (I'm) sending this email on behalf of John.

2: (I'm) sending this email on the request of John.
Comments 
1: (I'm) sending this email on behalf of John. I'm doing that for him.

2: (I'm) sending this email at the request of John.
I'm doing that for him. He asked me to do it.
I am sending this email to John's request to talk about...
Is that sentence wrong ? If yes, which my mistake you suppose is to ?
I am brazilian. Problem with the latin reference language ?
I am sending this email in response to John's request to talk about...

John has asked me to talk to you about...