Understanding Email Services

MUA: Mail User Agent (email client): The MUA is the front-end for the end user to manage and read stored email into the user's mailbox(es) and compose new email to be sent out via an MTA. This includes things like decoding MIME-encoded and encrypted messages. Other popular MUAs include Thunderbird, Kmail, evolution, Sylpheed, mulberry, pegasus, pine, and elm (mutt's predecessor).

MTA / SMTP / eSMTPSimple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission. It was last updated in 2008 with Extended SMTP [eSMTP] which is the protocol in widespread use today.

Features:

1. MTA  receives and sends mail to other remote MTAs. 
2. Routing emails.
3. Queuing emails.
4. Retrying mail sending if the next MTA doesn't accept email immediately sending a notification back to the original sender if the next MTA cannot handle the message.

Reasons for non-delivery:
1. Receiver's mailbox does not exist
2. Receiver's mailbox is full 
3. Receiver's mailbox or mail server unavailable 
4. Doesn't trust the remote MTA 
5. If after trying several times it cannot deliver the message it will normally report this back to the sender via a bounce message.

On the internet, MTAs between themselves communicate via SMTP, the Simple Mail Transfer Protocol. Some popular MTAs include:
http://www.exim.org/
http://www.postfix.org/
http://www.sendmail.org/
Qmail

SMTP Ports: 25, 587, 465

Commands to play with SMTP:
[root@hostname]# telnet mail.samnetworks.in 25
Trying 45.77.40.19...
Connected to mail.samnetworks.in.
Escape character is '^]'.
220 ESMTP Tue, 27 Mar 2018 09:18:42 -0400: UCE strictly prohibited

SSH Command line: 

lsof -i :25
lsof -i :465

[lsof with internet connections for port 25, 465]

-i [i] selects the listing of files any of whose Internet address matches the address specified in i. If no address is specified, this option selects the listing of all Internet and x.25 (HP-UX)
network files. 

Example:
[root@hostname]# lsof -i :25
COMMAND PID   USER FD TYPE DEVICE SIZE/OFF NODE NAME
exim          17315 exim 8u IPv4 242519 0t0 TCP hostname:49429->sb-in-f26.1e100.net:smtp (ESTABLISHED)

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