title: Why must SMTP User agents send mail through a mail server?
body: <p>My networking teacher said the SMTP messaging process from Alice to Bob goes through this process.</p>
  
  <p>(1) User agent -> (2) Alice's Mail server - > (3) Bob's mail server -> (4) Bob's User Agent</p>
  
  <p>Let's say Alice is on her User Agent (e.g. Thunderbird). Why does her email have to go through a mail server? Why couldn't she send a message directly to bob's mail server? If her user agent knows Bob's address (bob@gmail.com), couldn't her user agent do some sort of DNS lookup to figure out the IP address for gmail.com and send her SMTP message directly to Bob's mail server?</p>