How does it work
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DNS (Domain Name Server) (Example: domain.dom) Elements
(1) A record ---> IP Address #.#.#.# ("Dot-Quad", ".quad")
(2) cname ---> pointer to an A or another CNAME
(3) PTR ---> Name from .quad (#.#.#.#) to Domain Name (domain.dom)
(4) Mail Exchange record (MX Record): A Record Name
  (A) Multiple MX records allowed
  (B) Everything after the "@" sign is used to look for the MX record
  (C) in using DHCP, the A record is updated, without touching the MX 
      record MX Weight
    ( It should be noted that the DNS caching of the old A record 
      still applies so you still need to wait for those to expire.
      Most places solve this by placing a timeout value in minutes. )
    (i)   The weight listing decides which server has priority
    (ii)  lower numbers have higher priority
    (iii) Example:
          mx 10 domain.dom
          mx 20 domain.dom

Setting up server
-----------------
(1) Delivery Systems
  (A) local delivery (user on system)
  (B) virtual delivery (user on different system)
  (C) remote delivery
    (i) Mail Relay Methods
      (a) Open Relay (Very Dangerous)
          This is how spammers make their living
          "Spammers-R-Us"
          Never use this
          Always check for this
      (b) Authenticated Relay SMTP Auth
          Optionally use Trans. Layer Security (TSL) Only 
          (Using TLS is also known as Authentication via SMTPS)
      (c) Source IP Based Relay
          What IP are they connecting from?
      (d) Source Domain Based Relay (BAD BAD BAD unless combined)
          Example--> "from: xxx@domain.dom"
      (e) Destination Domain Based Relay
          Example--> "to: xxx@domain.dom"
      (f) Local Relay
          Anyone on local host can send email anywhere.
          Real-time Black-hole List (RBL) 
          - The original solution to the abuse of Open-Relay Mail Servers.

(2) Receiving Process
  (A) from the internet
  (B) server sees who mail is from
  (C) server sees who mail is to
  (D) server asks, do I handle this mail? (domain.dom)
  (E) e.mail id is looked up in Alias table. 
      NOC=Network Operations Center (noc -> someadmin)
      (Alias table can also direct to another domain.dom)
  (F) NOC is translated to User name (someadmin)
  (G) mail is delivered to user account on local computer 
      (to an "MBox file" such as /var/spool/mail/someadmin)(MDA)
  
  (News is not delivered in the same fashion as Mail)
  (Maildir format ---> /home/user-account/mail)
  See Todd Lyons's Sendmail Installation Tutorial (covers sendmail v8.11.2)

(3) Tools
  (A) Mail Transport Agent (MTA)
    (i)   SMTP
    (ii)  LMTP
    (iii) UUCP
  (B) Mail Delivery Agent (MDA)
    (i)  Procmail
    (ii) Cyrus IMAP
  (C) Mail User Agent (MUA)
    (i)   elm
    (ii)  pine
    (iii) mail
  (D) MX Troubleshooting
    (i) dig
      Example--> "dig +trace domain.dom ns"
      (a) +trace
    (ii) host
      Example--> "host -t mx domain.dom"
      (a) -t (type)
      (b) mx
      (c) soa (start of authority)