How to Configure Gmail as a Sendmail email relay | How to Configuring Sendmail to Relay Emails using Gmail SMTP #linuxtopic

Configure Sendmail using Gmail SMTP Relay

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In this topic we will configure internal sendmail of linux to gmail as relay using our existing gmail account.

Basic Information :

Gmail:

smtp.gmail.com

OS:

CentOS 7 - 64bit

Hostname:

lokesh.linuxtopic.com

192.168.156.100

Require Packages:

sendmail

mailx

Configure Postfix as Gmail SMTP Relay - Click Here..

Configure Sendmail as Gmail SMTP Relay 

To Install Sendmail package

yum install sendmail sendmail-cf cyrus-sasl-plain cyrus-sasl-md5 mailx

Configuration file path and file

cd /etc/mail/

Generate certificate for sendmail

cd /etc/pki/tls/certs
make sendmail.pem

Configuration Gmail Authentication

We will create auth file, that will store the authentication details to my Gmail account. if you dont have gmail Click here to create 

To Create a directory called my-auth inside /etc/mail to keep this file separate form other files and set the proper permission 

mkdir /etc/mail/my-auth

chmod 700 /etc/mail/my-auth

To create gmail-info file and type the following lines and save

vi /etc/mail/my-auth/gmail-info

AuthInfo:smtp.gmail.com "U:lokesh" "I:linuxtopic2015@gmail.com" "P:wwwwxxxxyyyyzzzz"
AuthInfo:smtp.gmail.com:587 "U:lokesh" "I:linuxtopic2015@gmail.com" "P:wwwwxxxxyyyyzzzz"

Note: In the above we used gmail SMTP Server with 587 Port and email id which we want to send email from linux box with password, we do not use actual password here generate 16 character app password and use here. 

how to generate app password or Collect App Password for GMAIL

Click Here...

Create gmail-info.db file 

cd /etc/mail/my-auth
makemap -r hash gmail-info.db < gmail-info
chmod 600 gmail-info gmail-info.db

Configure Sendmail :

We will create a backup file before the configure sendmail to send email via email SMTP Relay, and then we will add below line

Backup:

cp /etc/mail/sendmail.mc /etc/mail/sendmail.mc-`date -I`

Configure SMTP Relay

Open sendmail.mc file, find the line "MAILER(smtp)dnl" and add below line above the match.

vi  /etc/mail/sendmail.mc

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

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

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

define(`confAUTH_OPTIONS', `A p')dnl

TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl

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

FEATURE(`authinfo',`hash -o /etc/mail/my-auth/gmail-info.db')dnl

After adding look like:

To re-build the sendmail configuration

make -C /etc/mail

To allow SMTP service in firewall

firewall-cmd --permanent --add-service=smtp

Now we did all the required configuration and going to restart the sendmail service, make sure postfix service is disabled and stopped

To stop and disable if postfix is running 

systemctl stop postfix && systemctl disable postfix

To start and enable sendmail 

systemctl enable sendmail && systemctl restart sendmail

To send test email

We can test sendmail using mail command below

echo "This is sendmail configuration using Gmail SMTP Relay by LinuxTopic" | mail -s "message" lokesh@linuxtopic.com

Verify from email

To check logs

For troubleshooting we can check logs as well

tail -f /var/log/maillog

Thanks you !!

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