This option tells Dovecot where to find the mail server's SSL Key.
Edit /etc/dovecot/conf.d/10-ssl.conf and add or correct the
following line (note: the path below is the default path set by the
Dovecot installation. If you are using a different path, ensure you
reference the appropriate file):
ssl_key = </etc/pki/dovecot/private/dovecot.pem