GNU/Linux - sendmail to send emails from CLI or scripts

Configure Exim MTA

I personnaly have my own mail server but I will show here how to set Exim working as a smarthost with a gmail account.

Exim

• Edit /etc/exim4/update-exim4.conf.conf :

dc_eximconfig_configtype='smarthost'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1 ; ::1'
dc_readhost=''
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='smtp.gmail.com:587'
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='false'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'

• Edit /etc/exim4/passwd.client and set email address and password :

*:std_2021@gmail.com:myGMAILpassword123

Sending emails

Now that our Exim smarthost is correctly set we can use our sendmail command.

Simple

• Create a /tmp/mail.header file and write header :

To: user@shebangthedolphins.net
From: std_2021@gmail.com
Subject: test mail

Hello human world!
This message has been sent with sendmail!

• Send the mail :

user@host:~$ cat /tmp/mail.header | sendmail -f std_2021@gmail.com user@shebangthedolphins.net

Send mails from a file list

• Create /tmp/mail.list file :

user01@shebangthedolphins.net
user02@shebangthedolphins.net
user03@shebangthedolphins.net

• Create a /tmp/mail.header.origin file with a temp@address wich will be replaced later :

To: temp@address
From: std_2021@gmail.com
Date: Thu, 17 Jun 2021 08:45:25 +0200
Subject: test mail

• Create a /tmp/body :

Hi, this message has been sent with sendmail!

• Send emails :

user@host:~$ for i in $(cat /tmp/mail.list | sort -R | head -n 1); do sleep 20; cp /tmp/mail.header.origin /tmp/mail.header; echo "sending mail to $i"; sed -i "s/temp@address/$i/" /tmp/mail.header; sed -i "s/Date:.*/Date: $(date -R)/" mail.header; cat /tmp/mail.header /tmp/body | /usr/sbin/sendmail -f std_2021@gmail.com "$i"; done

• Same but easier to read :

for i in $(cat /tmp/mail.list | sort -R | head -n 1); do
	sleep 20s					#wait 20 second beetween each mail
	cp /tmp/mail.header.origin /tmp/mail.header	#copy template header to current header
	echo "sending mail to $i" 	#echo current recipient
	sed -i "s/temp@address/$i/" /tmp/mail.header		#replace temp@address by current recipient mail address in /tmp/mail.header 
	sed -i "s/Date:.*/Date: $(date -R)/" /tmp/mail.header		#replace Date field by current date in /tmp/mail.header
	cat /tmp/mail.header /tmp/body | /usr/sbin/sendmail -f std_2021@gmail.com "$i" #send email to current recipient
done

Send mails from a file list

user@host:~$ for i in $(seq 1 60); do sleep 4h; for i in $(cat /tmp/mail.list.txt | sort -R | head -n 1); do cp /tmp/mail.header.origin /tmp/mail.header; echo "sending mail to $i"; sed -i "s/temp@shebangthedolphins.net/$i/" /tmp/mail.header; grep Date mail.header | sed -i "s/Date:.*/Date: $(date -R)/" mail.header; cat /tmp/mail.header /tmp/body | /usr/sbin/sendmail -f user@shebangthedolphins.net "$i"; done; done

/tmp/mail.header
X-Priority: 1 (Highest)
From: sender@shebangthedolphins.net
To: temp@shebangthedolphins.net
Date: Thu, 17 Jun 2021 08:45:25 +0200
Subject: STD : Subject that I want
Content-type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
IME-Version: 1.0
Disposition-Notification-To: <sender@shebangthedolphins.net>

cat /tmp/body 
Hello human world!

Headers tricks

If needed, we can add some headers :

• Define content type and charset :

Content-type: text/plain; charset=utf-8

• Set Mime-Version (Normally automatically added) :

Mime-Version: 1.0

• Set highest priority :

X-Priority: 1 (Highest)

• Enable notification, to notify the sender about the ultimate disposition of the message :

Disposition-Notification-To: <sender@shebangthedolphins.net>

Send an email with an attachment

• Convert the file to be sent to base64 :

user@host:~$ base64 secret.svg 
PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSI5MDAiIGhlaWdo
dD0iNjAwIj48cGF0aCBmaWxsPSIjRUQyOTM5IiBkPSJNMCAwaDkwMHY2MDBIMHoiLz48cGF0aCBm
aWxsPSIjZmZmIiBkPSJNMCAwaDYwMHY2MDBIMHoiLz48cGF0aCBmaWxsPSIjMDAyMzk1IiBkPSJN
CAwaDMwMHY2MDBIMHoiLz48L3N2Zz4=

• Edit /tmp/body :

--19032019ABCDE
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit

Hi, this message has been sent with sendmail!

--19032019ABCDE
Content-Type: application/svg; name="secret.svg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="secret.svg"

PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSI5MDAiIGhlaWdo
dD0iNjAwIj48cGF0aCBmaWxsPSIjRUQyOTM5IiBkPSJNMCAwaDkwMHY2MDBIMHoiLz48cGF0aCBm
aWxsPSIjZmZmIiBkPSJNMCAwaDYwMHY2MDBIMHoiLz48cGF0aCBmaWxsPSIjMDAyMzk1IiBkPSJN
CAwaDMwMHY2MDBIMHoiLz48L3N2Zz4=
--19032019ABCDE

• Edit /tmp/header :

To: user@shebangthedolphins.net
From: std_2021@gmail.com
Date: Thu, 17 Jun 2021 08:45:25 +0200
Subject: test mail
Content-type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
IME-Version: 1.0
Content-Type: multipart/mixed; boundary="19032019ABCDE"

• Send email :

user@host:~$ cat /tmp/mail /tmp/mail | sendmail -f std_2021@gmail.com user@shebangthedolphins.net