[root@samba1 ~]# openssl s_client
-connect mail.example.com:587
-starttls smtp
-CAfile /etc/pki/tls/mail.example.com.pem

Nov 25 19:23:05 mail postfix/smtpd[5659]: initializing the server-side TLS engine
Nov 25 19:23:05 mail postfix/smtpd[5659]: connect from samba1.example.com[192.168.0.12]
Nov 25 19:28:05 mail postfix/smtpd[5659]: timeout after UNKNOWN from samba1.example.com[192.168.0.12]

account default {
set smtp=smtps://mail.example.com:587

#Authentication
set smtp-auth=login
set smtp-auth-user=my.email@example.com
set smtp-auth-password="my_password"

#Encryption
set smtp-use-starttls
set nss-config-dir=/etc/pki/nssdb
}

[root@samba1 ~]# certutil -L -d /etc/pki/nssdb

Certificate Nickname                        Trust Attributes
                                            SSL,S/MIME,JAR/XPI

mail.example.com.crt                        CT,,