Sendmail – LetsEncrypt and verify=OK

How To Configure LetsEncrypt and Sendmail Properly

This documentation pulls from a form post on FreeBSD from user Kuli.
https://forums.freebsd.org/threads/sendmail-and-letsencrypt.57675/

LetsEncrypt certificates aren’t listed in the main distributed ca-bundle.crt. The confCACERTand confCACERT_PATH configurations are two independent configurations that don’t really have anything to do with each other. When verifying certificates, it looks in the confCACERT_PATH for individual hashed files of root certificates. The confCACERTwill be configured with the intermediary LetsEncrypt chain.pem. Sendmail will then be happy to verify=OK the certificates. Do note that, it appears the majority of mail servers are using certificates that can’t be verified. So you will see a lot of NO. It’s better than FAIL. The script works with Fedora and probably any variant of Linux. Your experience may vary.

Sendmail Configuration

define(CERT_DIR',/etc/letsencrypt/live/<your site>')
define(confCACERT_PATH',/usr/local/etc/ssl/ROOT')
define(confCACERT',CERT_DIR/chain.pem')
define(confSERVER_CERT',CERT_DIR/cert.pem')
define(confSERVER_KEY',CERT_DIR/privkey.pem')
define(confCLIENT_CERT',CERT_DIR/cert.pem')
define(confCLIENT_KEY',CERT_DIR/privkey.pem')

Create the CACERT_PATH files

#!/bin/sh
#Separate the root cert into files each with only one cert and name hashed

RCert=/etc/ssl/certs/ca-bundle.crt
DESTDIR=/usr/local/etc/ssl/ROOT
mkdir -p $DESTDIR
cd $DESTDIR
rm -f *
cat $RCert | sed -E '/^(Certificate:|SHA1 Fingerprint|#| |$)/d' | awk 'BEGIN {c=0;} /BEGIN TRUS/{c++} { print > "cert." c ".pem"}'

for a in ls $DESTDIR
do
    mv $a openssl x509 -hash -noout < $a.0
done