The path of an email

The idea

Follow one email from aliceLaptop to bob's inbox, server by server. Each domain runs its own mail server, mailNorth for north.lab and mailSouth for south.lab: postfix speaks SMTP, dovecot speaks IMAP, and bob reads on bobLaptop.

Steps

1. The MX record names a domain's mail server

   A sender knows only an address like bob@south.lab, never which machine takes south.lab's mail. So a mail server asks DNS where mail for the domain goes. Ask the same question from aliceLaptop.

   aliceLaptop:~# dig MX south.lab +noall +answer
   aliceLaptop:~# dig +short south.lab
   aliceLaptop:~# dig +short mailSouth.lab

2. SMTP typed by hand

   Every mail server in the world must understand every other, so SMTP stays simple: text lines and three-digit replies on Port 25. Hand mailNorth a message for bob, line by line, and read each reply before the next.

   aliceLaptop:~# nc mailNorth.lab 25