First of all, we need to order a certificate , this certificate is the same with normal web server certificate. The certificate will have 3 files: cert, key, cabundle.
We use CentOS for example in below tutorial, please adjust the file to correct one on your server according to above description.
Postfix (SMTP server)
We can use postconf command to update SSL related settings directly:
postconf -e smtpd_tls_cert_file='/etc/pki/tls/certs/cert.pem'
postconf -e smtpd_tls_key_file='/etc/pki/tls/private/privkey.pem'
postconf -e smtpd_tls_CAfile='/etc/pki/tls/certs/fullchain.pem'
Restarting Postfix service is required.
Dovecot (POP3/IMAP server)
SSL certificate settings are defined in Dovecot main config file, /etc/dovecot/dovecot.conf (Linux/OpenBSD) or /usr/local/etc/dovecot/dovecot.conf (FreeBSD):
ssl = required
ssl_cert = Restarting Dovecot service is required.