Sendmail Relay Configuration

Intro

In this article I will go through the process of setting up Sendmail to relay email to MailJet service.

There are several options to setup relaying on your web hosting service, and also several providers that you can consider.

Incorporating the SMTP relay service with Mailjet allows to take advantage of other services provided such and Campaign management.

Requirements

For this setup you will need to have access to your server and permissions to install software.
Create one account on MailJet service
Have permissions to change your domain DNS records

MailJet

For this setup we are considering MailJet service but you can use a different one.
Depending on the tier level, you will have different limitations.

The Free tier allows:

200 emails per day
1500 contacts
6000 emails p/month

It is a good point to start and later increase if it makes sense.

DNS

SPF & DKIM are authentication systems that tell Internet Service Providers (ISPs), like Gmail and Yahoo, that incoming mail has been sent from an authorized system, and that it is not spam or email spoofing. To set Mailjet as an authorized sender and improve your deliverability, you need to modify your DNS records to include DKIM signature and SPF.

This document provides more detailed information

But basically you will need to include 2 TXT records

type: TXT , host: @ , value: “v=spf1 include:spf.mailjet.com ~all”

If you run a DNS query on your domain for TXT you need to see that info

dig -t TXT yourdomain.com

You also need to include the DKIM record follow the instructions provided

https://app.mailjet.com/account/sender?type=auth

There is one option to validate if the configuration is working properly

Add Domains

You will also need to configure the allowed domains that will be allowed and validate senders.

In the following URL you can make those:

https://app.mailjet.com/account/sender?type=domain

API Keys

The last step would be to create an API key for your service.

Go to following URL and create a new key, note it down as it will be required later.

https://app.mailjet.com/account/apikeys

Ok, now let’s configure our MTA

Configure Sendmail

For this setup you will need access to your hosting service and capable of installing software.

The following instructions are for a Ubuntu base distribution.

Install packages

sudo apt-get install sendmail

Configuration

In this setup we will configure to relay via SMTP all email using auth provided by the service

Start by editing the following file /etc/mail/sendmail.mc and add the following content at the end

dnl # Default Mailer setup
MAILER_DEFINITIONS
define(`SMART_HOST', `in-v3.mailjet.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/authinfo/smtp-auth.db')dnl
MAILER(`local')dnl
MAILER(`smtp')dnl

We need to setup authentication, remember the previous API key that you created you will need to include the information associaded to API_KEY and API_SECRET on the following file /etc/mail/authinfo/smtp-auth

AuthInfo: "U:root" "I:API_KEY" "P:API_SECRET"

sudo mkdir /etc/mail/authinfo
sudo nano /etc/mail/authinfo/smtp-auth

Example:

AuthInfo: "U:root" "I:1233450786523741256e" "P:ety555qtfgdghsd88wrfer"

After this you need to run the following command to update the service configuration
files

make -C /etc/mail

And restart sendmail service

systemctl restart sendmail

Test

In order to test you can execute the following command

echo "Test Email" | mail -s "Subject Here" recipient@example.com

You can now check in MailJet Stats session if your mail pass there.

Troubleshooting

You can check with the mailq command to understand if there is mail being block and the logs in /var/log/mail.log to understand if there is some issue.

Conclusion

In this article we went though the configuration of Sendmail service to relay emails through the Mailjet service. It covers the necessary configurations in both DNS and the Mailjet service to ensure seamless email delivery from your web hosting server.

18 Feb, 2024 - About 3 minutes 