Step 1: Install Exim
Install Exim from the CentOS repository using the following command line:
sudo yum install exim
After the installation is complete, you can check the installed Exim version with the command:
exim --version
result:
[root@tutorialspots ~]# exim --version Exim version 4.91 #2 built 22-Aug-2018 14:16:00 Copyright (c) University of Cambridge, 1995 - 2018 (c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007 - 2018 Berkeley DB: Berkeley DB 5.3.21: (May 11, 2012) Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc OpenSSL Content_Scanning DKIM DNSSEC Event OCSP PRDR TCP_Fast_Open Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmjz dbmnz dnsdb dsearch ldap ldapdn ldapm nis nis0 nisplus passwd sqlite Authenticators: cram_md5 cyrus_sasl dovecot gsasl plaintext spa tls Routers: accept dnslookup ipliteral manualroute queryprogram redirect Transports: appendfile/maildir/mailstore/mbx autoreply lmtp pipe smtp Malware: f-protd f-prot6d drweb fsecure sophie clamd avast sock cmdline Fixed never_users: 0 Configure owner: 0:0 Size of off_t: 8 Configuration file is /etc/exim/exim.conf
Step 2: Configure Exim
Configure Exim via configuration file /etc/exim/exim.conf or /etc/exim/exim4.conf (you can see in step 1). You can edit this file using a text editor such as vi or nano.
Add disable_ipv6 = true at the top of config file to disable ipv6 if need
After editing the configuration, you need to restart Exim to apply the changes:
sudo systemctl restart exim
Step 3: Install and configure DKIM
Install opendkim:
sudo yum install opendkim
[root@tutorialspots ~]# sudo yum install opendkim Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: mirror.viettelcloud.vn * epel: repo.jing.rocks * extras: mirror.viettelcloud.vn * remi-php56: ftp.riken.jp * remi-safe: ftp.riken.jp * rpmforge: kartolo.sby.datautama.net.id * updates: mirror.viettelcloud.vn http://yum.mariadb.org/10.0/centos7-amd64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found Trying other mirror. To address this issue please refer to the below wiki article https://wiki.centos.org/yum-errors If above article doesn't help to resolve this issue please use https://bugs.centos.org/. Resolving Dependencies --> Running transaction check ---> Package opendkim.x86_64 0:2.11.0-0.1.el7 will be installed --> Processing Dependency: libopendkim(x86-64) = 2.11.0-0.1.el7 for package: opendkim-2.11.0-0.1.el7.x86_64 --> Processing Dependency: libbsd.so.0(LIBBSD_0.0)(64bit) for package: opendkim-2.11.0-0.1.el7.x86_64 --> Processing Dependency: libbsd.so.0()(64bit) for package: opendkim-2.11.0-0.1.el7.x86_64 --> Processing Dependency: libmilter.so.1.0()(64bit) for package: opendkim-2.11.0-0.1.el7.x86_64 --> Processing Dependency: libopendbx.so.1()(64bit) for package: opendkim-2.11.0-0.1.el7.x86_64 --> Processing Dependency: libopendkim.so.11()(64bit) for package: opendkim-2.11.0-0.1.el7.x86_64 --> Running transaction check ---> Package libbsd.x86_64 0:0.12.2-1.el7 will be installed --> Processing Dependency: libmd.so.0(LIBMD_0.0)(64bit) for package: libbsd-0.12.2-1.el7.x86_64 --> Processing Dependency: libmd.so.0()(64bit) for package: libbsd-0.12.2-1.el7.x86_64 ---> Package libopendkim.x86_64 0:2.11.0-0.1.el7 will be installed ---> Package opendbx.x86_64 0:1.4.6-6.el7 will be installed ---> Package sendmail-milter.x86_64 0:8.14.7-6.el7 will be installed --> Running transaction check ---> Package libmd.x86_64 0:1.1.0-1.el7 will be installed --> Finished Dependency Resolution Dependencies Resolved ================================================================================ Package Arch Version Repository Size ================================================================================ Installing: opendkim x86_64 2.11.0-0.1.el7 epel 222 k Installing for dependencies: libbsd x86_64 0.12.2-1.el7 epel 130 k libmd x86_64 1.1.0-1.el7 epel 41 k libopendkim x86_64 2.11.0-0.1.el7 epel 75 k opendbx x86_64 1.4.6-6.el7 epel 46 k sendmail-milter x86_64 8.14.7-6.el7 base 71 k Transaction Summary ================================================================================ Install 1 Package (+5 Dependent packages) Total download size: 585 k Installed size: 1.4 M Is this ok [y/d/N]: y Downloading packages: (1/6): libbsd-0.12.2-1.el7.x86_64.rpm | 130 kB 00:01 (2/6): libmd-1.1.0-1.el7.x86_64.rpm | 41 kB 00:00 (3/6): libopendkim-2.11.0-0.1.el7.x86_64.rpm | 75 kB 00:00 (4/6): opendbx-1.4.6-6.el7.x86_64.rpm | 46 kB 00:00 (5/6): opendkim-2.11.0-0.1.el7.x86_64.rpm | 222 kB 00:00 (6/6): sendmail-milter-8.14.7-6.el7.x86_64.rpm | 71 kB 00:00 -------------------------------------------------------------------------------- Total 164 kB/s | 585 kB 00:03 Running transaction check Running transaction test Transaction test succeeded Running transaction Installing : libmd-1.1.0-1.el7.x86_64 1/6 Installing : libbsd-0.12.2-1.el7.x86_64 2/6 Installing : libopendkim-2.11.0-0.1.el7.x86_64 3/6 Installing : sendmail-milter-8.14.7-6.el7.x86_64 4/6 Installing : opendbx-1.4.6-6.el7.x86_64 5/6 Installing : opendkim-2.11.0-0.1.el7.x86_64 6/6 Verifying : opendbx-1.4.6-6.el7.x86_64 1/6 Verifying : libbsd-0.12.2-1.el7.x86_64 2/6 Verifying : sendmail-milter-8.14.7-6.el7.x86_64 3/6 Verifying : libopendkim-2.11.0-0.1.el7.x86_64 4/6 Verifying : opendkim-2.11.0-0.1.el7.x86_64 5/6 Verifying : libmd-1.1.0-1.el7.x86_64 6/6 Installed: opendkim.x86_64 0:2.11.0-0.1.el7 Dependency Installed: libbsd.x86_64 0:0.12.2-1.el7 libmd.x86_64 0:1.1.0-1.el7 libopendkim.x86_64 0:2.11.0-0.1.el7 opendbx.x86_64 0:1.4.6-6.el7 sendmail-milter.x86_64 0:8.14.7-6.el7 Complete!
Once installed, you need to generate a key pair for DKIM:
sudo opendkim-genkey -s
example: sudo opendkim-genkey -s postmaster -d tutorialspots.com
This command will generate 2 files in current directory: postmaster.private and postmaster.txt
Move the private key file into the folder /etc/opendkim/keys
sudo mv postmaster.private /etc/opendkim/keys
Add Exim user to group opendkim:
sudo usermod -aG opendkim exim
Chown private key:
chown opendkim:opendkim /etc/opendkim/keys/postmaster.private
Edit the DKIM configuration file /etc/opendkim.conf:
sudo nano /etc/opendkim.conf
Add or edit the following lines:
Domain tutorialspots.com KeyFile /etc/opendkim/keys/postmaster.private Selector postmaster
Edit the Exim configuration file to use DKIM:
sudo nano /etc/exim/exim.conf
Add the following line to the section TRANSPORT CONFIGURATION:
add after:
remote_smtp:
driver = smtp
message_size_limit = ${if > {$max_received_linelength}{998} {1}{0}}
dkim_domain = tutorialspots.com dkim_selector = postmaster dkim_private_key = /etc/opendkim/keys/postmaster.private
Restart Exim and opendkim to apply the changes:
sudo systemctl restart exim opendkim
Step 4: Configure DNS
Finally, you need to add the DKIM record to your domain’s DNS configuration file.
Open file postmaster.txt to see content:
[root@tutorialspots ~]# cat postmaster.txt
postmaster._domainkey IN TXT ( "v=DKIM1; k=rsa; "
"p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDr1bP/ttIeGV/G2Tcqoci0HrbfGP/J4ghANKnrDKaqRO0UNXzo8SB7MaZ3NgK5zY0vk0BqCbs5buJxotpEpjDD8gwnWjqaZZwnKmoL9ZT8G9D24WyjlEhp3QNx6TA8nywpnT7KJQqrfyI/71TmJdzQIKq3HMABJhmVf1Y2/ZnzqQIDAQAB" ) ; ----- DKIM key postmaster for tutorialspots.com
Here’s an example of how to add a DKIM record to DNS:
postmaster._domainkey.tutorialspots.com. IN TXT "p=<public key>"
Replace
postmaster.txt .
Step 5: SPF configuration:
Add SPF Record to DNS :
Log in to your domain’s DNS admin account. Usually through the administration interface of the domain name provider or DNS service you are using.
Add a new TXT record with the following content:
v=spf1 a:mail.yourdomain.com ip4:<server_ip> include:_spf.google.com ~all
example
v=spf1 a:mail.yourdomain.com ip4:11.11.11.11 include:_spf.google.com ~all
Save changes.
read more about SPF: https://support.google.com/a/answer/10685031?sjid=14440567733282704103-AP
Done!
Verify SMTP server
[root@tutorialspots ~]# telnet 127.0.0.1 25 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'. 220 google.com ESMTP Exim 4.91 Mon, 29 Apr 2024 20:06:14 +0700
Test:
yum install mailx
echo "Test mail from EXIM" | mail -s "Test EXIM" -f "postmaster@yourdomain.com" youremail@gmail.com
