A while back, I noticed a recurring error in my maillog that read:

postfix/smtp: certificate verification failed for gmail-smtp-in.l.google.com[74.125.53.27]:25: untrusted issuer /C=US/O=Equifax/OU=Equifax Secure Certificate Authority

If you’re seeing this too, there’s an easy fix.

The problem stems from the fact that Google changed certificate providers from Thawte to Equifax, and your mail system doesn’t recognize the Equifax certificate authority as valid. The solution is to add a copy of the Equifax certificate to Postfix’s local root certificate store. And while we’re at it, we may as well add Thawte’s as well.

Step 1: Back up your original root certificate store

Before messing with any of the settings, do the following to backup your original root certificate:

cd /etc/postfix/ssl
cp cacert.pem cacert.pem.bak

Step 2: Create local copies of Equifax and Thawte certificates

Using your favorite text editor, create a file called Equifax_Secure_CA.pem in the /etc/postfix/ssl directory. Paste the following into the file:

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Next, create a file called Thawte_Premium_Server_CA.pem in the /etc/postfix/ssl directory and paste the following into that file:

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Step 3: Add the Equifax and Thawte certificates into your local root certificate file

Add the two new certificates into your local root certificate file with:

cat /etc/postfix/ssl/Equifax_Secure_CA.pem >> /etc/postfix/ssl/cacert.pem
echo >> /etc/postfix/ssl/cacert.pem
cat /etc/postfix/ssl/Thawte_Premium_Server_CA.pem >> /etc/postfix/ssl/cacert.pem

(the second command above adds a line break between the two certs in your local root cert file)

Step 4: Restart Postfix

Restart Postfix with:

service postfix restart

Send a message through your SMTP server to a Gmail test address and you should no longer see those errors in your maillog!