Configuring Sendmail with a Smart Host and SMTP Authentication
In this blog post, we’ll examine how to edit the sendmail. mc file to configure Sendmail to relay mail through an external SMTP service, such as SMTP2GO.
This setup will be userfull when:
-
You want to send emails through a third-party SMTP provider.
-
Your server is blocked from sending mail directly on port 25.
-
You must authenticate with your SMTP provider using a specific port (e.g., 2525).
🔧 Step-by-Step Sendmail Configuration
Here’s a snippet from the sendmail.mc
file:
#dnl define(`SMART_HOST', `smtp.your.provider')dnl
define(`SMART_HOST', `mail.smtp2go.com')dnl
FEATURE(`authinfo')dnl
define(`RELAY_MAILER_ARGS', `TCP $h 2525')dnl
dnl #
Let’s break it down:
Explanation of Each Line
1. Smart Host Definition
#dnl define(`SMART_HOST', `smtp.your.provider')dnl
This is a commented-out placeholder. You can use this format to set a different SMTP server if needed.
2. Actual Smart Host in Use
define(`SMART_HOST', `mail.smtp2go.com')dnl
This tells Sendmail to route all outbound mail through SMTP2GO.
3. Authentication Info Feature
FEATURE(`authinfo')dnl
This enables authentication support, allowing you to send email through a secure SMTP server that requires login credentials.
4. Relay Mailer Arguments
define(`RELAY_MAILER_ARGS', `TCP $h 2525')dnl
This specifies the port to use when connecting to the SMTP server. Here, we're using port 2525, a common alternative to port 25 (which is often blocked by ISPs).
5. Final Comment Line
dnl #
A dummy comment or placeholder, no operational effect.
Next Steps After Configuration
Once your sendmail.mc
file is configured:
-
Rebuild sendmail configuration
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
-
Create the authinfo file Add your SMTP credentials in
/etc/mail/authinfo
:AuthInfo:mail.smtp2go.com "U:root" "I:your_username" "P:your_password"
-
Hash the authinfo file
makemap hash /etc/mail/authinfo < /etc/mail/authinfo
-
Restart sendmail
systemctl restart sendmail
Test the Configuration
Send a test email using mail
or sendmail
to confirm that everything is working.
echo "Test Email from Sendmail via SMTP2GO" | mail -s "Sendmail Test" your@email.com
Final Thoughts
Using a smart host like SMTP2GO makes your email delivery more reliable and helps avoid issues with ISPs blocking direct mail traffic. With a few lines in sendmail.mc
, and some authentication setup, you're good to go!
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