email domain resolution

I am studying for the Network+ exam and ran across a practice question that was answered different in two places. I was hoping someone could give me the definitive answer to this question.

You have sent an e-mail to user@email.us. Your email address is me@mydomain.com. In what order are the domain names for this e-mail resolved.

A. com,us,email.us,mydomain.com
B. mydomain.com,com,us,email.us
C. com,mydomain.com,us,email.us
D. email.us,mydomain.com,com.us

Thanks in advance

 

My guess is C.

The DNS server always has to start at the highest level and work down from that. The heirachy works from highest at right to lowest at left. So before DNS can resolve mydomain.com, it must first resolve the root domain "com ". The first step must therefore be a call to a root server. So that rules out answers B and D.

DNS servers handle one request at a time. That is, it will either deal with the complete mydomain.com resolution then the email.us (or the other way round). So at the time it is resolving mydomain.com it will have already completed the email.us resolution or not have started it. Therefore the server will not start resolving part of the email.us name before it has completed the mydomain.com resolution. So therefore A cannot be right.

That leaves C: com,mydomain.com,us,email.us. That has both resolutions working in the right order (resolve root first, then daughter of root) and deals with the requests sequentially.