/etc/nginx/sites-available/ispconfig.vhost

ssl_certificate /etc/ssl/private/……….chain.pem;
ssl_certificate_key /etc/ssl/private/…………..key;

/etc/postfix/main.cf

# Pfade zu den Keys fuer den Mailempfang
smtpd_tls_cert_file = /etc/ssl/private/….pem
smtpd_tls_key_file = /etc/ssl/private/….key
smtpd_tls_CAfile = /etc/ssl/private/ca.pem