I've had my Xen-based server for just over a year now, which means that the certificates that permit SSL-encrypted email delivery and retrieval expired.
Generating new ones wasn't a problem:
sudo openssl req -new -x509 -nodes \ -out /etc/postfix/smtpd.cert \ -keyout /etc/postfix/smtpd.key \ -days 365 sudo openssl req -new -x509 -nodes \ -out /etc/ssl/certs/dovecot.pem \ -keyout /etc/ssl/private/dovecot.pem \ -days 365 sudo /etc/init.d/courier restart sudo /etc/init.d/postfix restart