add linux email service

snowme34 · Mar 6, 2019 · 52484dd · 52484dd
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diff --git a/cspell_dict_bash.txt b/cspell_dict_bash.txt
@@ -520,4 +520,13 @@ unicast
 multicast
 CSMA
 
-priv
+priv
+
+gmail
+SMTP
+linode
+sendmail
+procmail
+maildrop
+dovercot
+imaps
diff --git a/docs/source/index.rst b/docs/source/index.rst
@@ -108,6 +108,7 @@ The organization of this site is as following:
    reference/linux/linux-network-config
    reference/linux/introduction-to-logs-and-rsyslog
    reference/linux/linux-service
+   reference/linux/linux-email-service
 
 
 .. toctree::

diff --git a/docs/source/reference/linux/linux-email-service.md b/docs/source/reference/linux/linux-email-service.md
@@ -0,0 +1,193 @@
+# Linux Email Service
+
+The email services.
+
+Born in 1960s.
+
+Consists of many components. Not achieved by single service.
+
+For formal communications.
+
+## Basics
+
+Below is one possible email infrastructure.
+
+[schema](https://i.imgur.com/PVyVugV.png)
+
+### MxA
+
+MSA (Mail Submission Agent) will be ignored for simplicity.
+
+MUA
+
+* Mail User Agent
+* The client software, such as Thunderbird.
+* And the web email applications, such as Gmail web app.
+
+MTA
+
+* Mail Transfer Agent
+* Server-side program.
+*Implements SMTP, transfers the email.
+
+MDA
+
+* Mail Deliver Agent
+* When MTA receives new email, MDA will store the email to the server.
+
+MRA
+
+* Mail Receive Agent
+* Implements IMAP or POP3.
+* Communicates with clients, sends email to clients using IMAP or POP3.
+
+### Main Protocols
+
+SMTP
+
+* Simple Mail Transfer Protocol
+* All emails are sent using SMTP
+
+IMAP
+
+* Internet Message Access Protocol
+* Email clients use IMAP to fetch emails from server
+
+POP3
+
+* Post Office Protocol
+* Email clients use IMAP to fetch emails from server
+
+## MUA
+
+Email Client.
+
+The only component that users can interact directly with.
+
+Common ones:
+
+* outlook
+* mac mail
+* thunderbird
+* mutt
+
+Web-based MUA is more common: gmail
+
+## IMAP POP3
+
+The standard protocols used for communications between email clients and servers.
+
+MUA uses IMAP and POP3 to **retrieve** emails.
+
+Users can decide if they want to keep the emails in the server
+after retrieval. Decades ago it was crucial since the storage
+is limited, but today people don't care anymore.
+
+### Difference between IMAP and POP3
+
+IMAP will "sync" all the operations, including read/unread, star and so on.
+
+POP3 downloads all the emails. All the operations are local and they do
+not affect the ones in the server.
+
+Again, this is a question of old days. Due to stability and bandwidth limit,
+people in old days prefer POP3 since it was more stable. But today IMAP is
+recommended.
+
+## MTA SMTP
+
+The actual email server software component.
+
+Uses SMTP (simple email transfer protocol) to **send** emails.
+
+MUA uses SMTP to communicate with MTA.
+
+MTA is responsible for communication between different mail servers.
+
+Common MTA:
+
+* sendmail
+* postfix
+
+MTA is only responsible for *transferring* emails.
+
+## MDA
+
+Stores the emails received by MTA to the server.
+
+Usually MTA and MDA are closely related, but still separated.
+They can be treated as a whole.
+
+Path ([How can I find my local mail spool?](https://unix.stackexchange.com/questions/82910/how-can-i-find-my-local-mail-spool)):
+
+```bash
+/var/spool/mail/$USER
+```
+
+Why Separated from MTA
+
+* MDA can also filter the spam emails and scan the virus.
+* Please look this up on your own :)
+
+Common MDA
+
+* procmail
+* maildrop
+
+The default MDA of sendmail and postfix is procmail.
+
+## MRA
+
+Implements IMAP, POP3.
+
+Communicates with MUA. Will return the emails via IMAP or POP3 after
+MUA requests them.
+
+The only part that MUA interact with when retrieving emails.
+
+The reason why we need to provide 2 addresses for the MUA config
+
+* SMTP -> MTA
+* IMAP/POP3 -> MRA
+
+Common MRA
+
+* Dovercot
+
+Usually will encrypt the communication. So people today
+have 2 more protocols:
+
+* IMAPS
+* POP3S
+
+## Email Address
+
+There are 2 types of email addresses.
+
+```bash
+user@mail.web.net
+username@hostname-of-email-server
+
+user@web.net
+username@domain
+```
+
+The second one is more common.
+
+But it does not specify the email server (the actual server
+that processes emails).
+
+And MX record is used in such situation.
+
+Use DNS to lookup the MX record to get the actual address
+of the mail server.
+
+```bash
+dig -t mx gmail.com
+```
+
+## Read More
+
+* [Running a Mail Server - Linode](https://www.linode.com/docs/email/running-a-mail-server/)
+* [Extended SMTP - Wikipedia](https://en.wikipedia.org/wiki/Extended_SMTP)
+* [8.5. Setting Up Your System To Use E-Mail](https://www.debian.org/releases/stable/i386/ch08s05.html.en)
diff --git a/docs/source/reference/linux/linux-email-service.png b/docs/source/reference/linux/linux-email-service.png
diff --git a/docs/source/reference/linux/linux-email-service.xml b/docs/source/reference/linux/linux-email-service.xml
@@ -0,0 +1 @@
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diff --git a/travis_link_check_whitelist.txt b/travis_link_check_whitelist.txt
@@ -8,4 +8,5 @@ waifu2x.udp.jp
 www.pixivision.net
 dsa.cs.tsinghua.edu.cn
 https://huaji8.top/
-www.gnu.org
+www.gnu.org
+https://color.adobe.com/explore/