Mail system

There are three main functions that make up an e-mail system.

First there is the Mail User Agent (MUA) which is the program a user actually uses to compose and read mails.
Then there is the Mail Transfer Agent (MTA) that takes care of transferring messages from one computer to another.
And last there is the Mail Delivery Agent (MDA) that takes care of delivering incoming mail to the user's inbox.

Function	Name	Tool which do this
Compose and read	MUA (User Agent)	mutt, thunderbird
Transferring	MTA (Transfer Agent)	msmtp, exim4, thunderbird
Delivering incoming mail to user's inbox	MDA (Devliery agent)	exim4, thunderbird

MTA

It exists two types of MTA (Mail Transfert Agent)

Mail server : like postfix, or sendmail-server
SMTP client, which only forward to a SMTP relay : like ssmtp (deprecated since 2013), use mstmp instead,

Check what is your email sender, by looking at the sym link of sendmail

which sendmail
/usr/sbin/sendmail

ls -l /usr/sbin/sendmail
lrwxrwxrwx 1 root root 5 Jul 15  2014 /usr/sbin/sendmail -> ssmtp

In this case, ssmtp in my mail sender

Client

MSMTP (client config)

msmtp est un client SMTP très simple et facile à configurer pour l'envoi de courriels.
Son mode de fonctionnement par défaut consiste à transférer les courriels au serveur SMTP que vous aurez indiqué dans sa configuration. Ce dernier se chargera de distribuer les courriels à leurs destinataires.
Compatible :

sendmail ou bsd-mailx
Handle TLS
Multiples account
Multiple authentication method
Distribution distribution

Installation

apt install msmtp msmtp-mta

vim /home/baptiste/.msmtprc

hashtag Valeurs par défaut pour tous les comptes.
defaults
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        ~/.msmtp.log

hashtag Exemple pour un compte Gmail
account        gmail
host           smtp.gmail.com
port           587
from           username@gmail.com
user           username
password       plain-text-password

hashtag Définir le compte par défaut
account default : gmail

Send an email

Using bsd-mailx (prefered)

First, ensure you have your client (msmtp) configured /home/baptiste/.msmtprc
Get the binary mail

apt install bsd-mailx

echo "mon message ici" | mail -s "un super sujet ici" baptistedauphin76@gmail.com

Using sendmail

Test email sending

echo "mon message ici" | sendmail baptistedauphin76@gmail.com

You run the command... and, oops: sendmail: Cannot open mailhub:25. The reason for this is that we didn't provide mailhub settings at all. In order to forward messages, you need an SMTP server configured. That's where SSMTP performs really well: you just need to edit its configuration file once, and you are good to go.

Using NetCat (hardcore method)

Manually write each steps of the SMTP protocol.

nc smtp.free.fr 25

telnet smtp.free.fr 25
Trying 212.27.48.4...
Connected to smtp.free.fr.
Escape character is '^]'.
220 smtp4-g21.free.fr ESMTP Postfix
HELO test.domain.com
250 smtp4-g21.free.fr
MAIL FROM:<test@domain.com>
250 2.1.0 Ok
RCPT TO:<toto@domain.fr>
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
Subject: test message
This is the body of the message!
.
250 2.0.0 Ok: queued as 2D8FD4C80FF
quit
221 2.0.0 Bye
Connection closed by foreign host.

Mail