Reverse DNS for mailserver or hostname?

Here's what I don't understand about reverse dns for my mailserver:

The hostname of my machine is www.mydomain.com.
I have forward DNS setup for mail.mydomain.com.

1. Should I setup reverse DNS to point to mail.mydomain.com, or www.mydomain.com?

2. If the answer is www.mydomain.com, then is the mail.mydomain.com entry irrelevant and I should just remove it?

3. Am I supposed to somehow make sendmail send from mail.mydomain.com and if so, how?

 

 

 

 

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