[Complete Guide] How to Install Sendmail Server on Centos 7?

The author voluntarily contributed this tutorial as a part of Pepipost Write to Contribute program.

Introduction

Sendmail is a feature-rich MTA (Mail Transfer Agent) uses SMTP protocol for sending mail. Though Sendmail has been replaced by postfix in modern RHEL versions it is widely used in RHEL 5 or its earlier version. Sendmail is recommended by most of the system administrator as an MTA(Mail transfer agent) server over other MTAs.

Prerequisites Before you Install Sendmail Server on Centos 7 

Highly recommended to have root privileges in order to perform the below steps:

Step 1 

Add the centos 7 EPEL repositories, open terminal and paste the below command:

sudo yum install epel-release

Step 2

Install Sendmail with dependency from yum package manager

sudo yum install sendmail sendmail-cf m4

(Note: m4 is a macro processor you need to use to compile Sendmail configuration file. )

Once the installation is done, you will be getting output like this:

Transaction Summary
=================================================
Install 3 packages
Total download size: 1.2 M
Installed size: 3.1 M
Is this ok [y/d/N]: y
Downloading packages:
(1/3): sendmail-cf-8.14.7-5.el7.noarch.rpm                                                                                                        | 186 kB  00:00:00
(2/3): m4-1.4.16-10.el7.x86_64.rpm                                                                                                                | 256 kB  00:00:00
(3/3): sendmail-8.14.7-5.el7.x86_64.rpm                                                                                                           | 736 kB  00:00:01
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total                                                                                                                                    888 kB/s | 1.2 MB  00:00:01
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : m4-1.4.16-10.el7.x86_64                                                                                                                               1/3
  Installing : sendmail-8.14.7-5.el7.x86_64                                                                                                                          2/3
  Installing : sendmail-cf-8.14.7-5.el7.noarch                                                                                                                       3/3
  Verifying  : sendmail-8.14.7-5.el7.x86_64                                                                                                                          1/3
  Verifying  : sendmail-cf-8.14.7-5.el7.noarch                                                                                                                       2/3
  Verifying  : m4-1.4.16-10.el7.x86_64                                                                                                                               3/3
Installed:
  m4.x86_64 0:1.4.16-10.el7                          sendmail.x86_64 0:8.14.7-5.el7                          sendmail-cf.noarch 0:8.14.7-5.el7
Complete!

OR

rpm -qa | grep sendmail

Step 3: Configure Sendmail Server 

Before directly edit /etc/mail/sendmail.mc for configuration we need to understand important file existence in /etc/mail directory.

• access: allowing or denying other systems to use Sendmail for outbound emails.
• domaintable: used for domain name mapping for Sendmail.
• local-host-names: used to define an alias for a host.
• mailertable: used to override routing for particular domains.
• virtusertable: allowing multiple virtual domains to be hosted on one machine.

Make the following changes in sendmail.mc file below is the command:

vim /etc/mail/sendmail.mc

define(`SMART_HOST', `smtp.gmail.com')dnl 

Note: Set your SMTP hostname above 

#Add the below two lines in your sendmail.mc file to listen on port 465 and 587:

define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl 

#Uncomment the below lines:

TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl 

#Add the below lines:

FEATURE(`authinfo', `hash /etc/mail/auth/client-info.db')dnl
FEATURE(`access_db', `hash -T<TMPF> -o /etc/mail/access.db')dnl
DAEMON_OPTIONS(`Port=smtp, Name=MTA')dnl 

#Uncomment for Sendmail to listen on port 587

DAEMON_OPTIONS(`Port=submission, Name=MSA, M=Ea')dnl 

#Uncomment for Sendmail to listen on port 587

DAEMON_OPTIONS(`Port=smtps, Name=TLSMTA, M=s')dnl 

#Mention your domain and uncomment

MASQUERADE_AS(`mydomain.com')dnl

Save and exit the file.

Step 4: Make Auth Directory

By assigning proper permission to make auth directory inside /etc/mail

sudo mkdir /etc/mail/auth
cd /etc/mail/auth

Create a new file client-info and below auth in the file.

vim client-info
AuthInfo:gmail.com "U:username" "P:password" "M:PLAIN"
AuthInfo: "U:username" "P:Password" "M:PLAIN
makemap -r hash client-info.db < client-info

Add your relay IP

vi /etc/mail/access

Connect: [your-ip ]  RELAY

# Check the /usr/share/doc/sendmail/README.cf file for a description
# of the format of this file. (search for access_db in that file)
# The /usr/share/doc/sendmail/README.cf is part of the sendmail-doc
# package.
# If you want to use AuthInfo with "M:PLAIN LOGIN", make sure to have the
# cyrus-sasl-plain package installed.
# By default we allow relaying from localhost...
Connect:localhost.localdomain RELAY
Connect:localhost RELAY
Connect:127.0.0.1 RELAY 

Step 5: Recompiling Sendmail Configuration

Update Sendmail configuration by compiling the /etc/mail/sendmail.mc file using m4 macro processor.

sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

OR

sudo make -C /etc/mail

Once you have recompiled restart the service using below command.

sudo service sendmail restart

Step 6: Send Your First Email

sendmail -v myemailid@gmail.com

Common Errors and Exceptions

#Error 1:

Disabling HOST statistics file(/var/lib/sendmail/host_status).
Creating /etc/mail/sendmail.cf...
Creating /etc/mail/submit.cf...
Informational: confCR_FILE file empty: /etc/mail/relay-domains
Informational: confCT_FILE file empty: /etc/mail/trusted-users
Updating /etc/mail/access...
Updating /etc/mail/aliases...
WARNING: local host name (ixtmixilix) is not qualified; see cf/README: WHO AM I?

This issue occurs when your hostname/domain name is not mapped with your server.  It's a warning, that your hostname won’t work outside your local network.  

You can simply set your hostname to a registered domain by using below command:

hostnamectl set-hostname my.new-hostname.server

#Error 2: Sendmail Error 550 5.1.1 “User Unknown”

‘550 5.1.1 User unknown’ is the most common issue which occurs while sending mails to Exchange/Recipient servers. 550 error means there is a problem with the recipient server due to which such emails are not delivered. Such issues arise not only with the Sendmail server but with postfix, Exim, etc.

Possible reasons for the occurrence of this issue:

1. Wrong Recipient address

• The sender would get a bounce message "550 5.1.1 User unknown" if the recipient mailbox is not available.
• This occurs due to human error while subscribing to the mailing list or if you have purchased an email list (this is a bad practice that can result in a downgrade of your IP reputation).
• Recipient mailbox is moved or deleted from the recipient mail server.
• A misconfiguration or bug on the recipient mail server can cause the unavailability of recipient mail account

How to avoid it?
Always validate email id by sending an email confirmation before sending your regular updates.
 

2. DNS setting is incorrect for domain

• 550 error can also occur if MX entry is not resolving correctly or DNS settings are incorrect e.g. MX for a domain, xyz.com is resolving some other domains MX.
• In the above case, the recipient account will be unavailable due. so if you are sending an email to example@gmail.com and it is resolving wrong MX then it will try sending mail to example@something.com

How to avoid it?
Check your MX record by using dig from the command line:

dig example.com MX

Note: If dig is not installed try using;

sudo apt-get install dig -y
sudo yum install dig -y
sudo dnf install dig -y
 

3. Email filters set for the domain

• These filters are mostly applied to cooperate mailboxes. Also, if the recipient email account has a forwarder set and that is incorrectly set to a nonexisting account it will result in 550 error.
• Filtering for incoming mails using email filters or anti-spam firewalls can end up bouncing emails from certain sender servers.

How to avoid?
Check your outgoing firewalls and rules, if any. If all good, contact recipient server support and get the email filters verified for any erroneous filters or rules that can be blocking mails.

Hope you liked this article, where the A-Z of Sendmail is explained in simple language. Using these simple steps you can install Sendmail Server on Centos.

In case you are facing some issues which are not listed above in the tutorial, or you have some suggestions, then please feel free to contribute below in comments.