Publish the intermediate CA.

templates/main.cf.j2

@@ -13,6 +13,7 @@ readme_directory = no
 # Server
 {% if letsencrypt_acme_install is defined %}
 {% if postfix_use_letsencrypt %}
+smtpd_tls_CAfile={{ letsencrypt_acme_certs_dir }}/fullchain
 smtpd_tls_cert_file={{ letsencrypt_acme_certs_dir }}/cert
 smtpd_tls_key_file={{ letsencrypt_acme_certs_dir }}/privkey
 {% else %}