Perhaps you have changed the path of your certificates. Check this older answer:
Something like
# file /etc/postfix/main.cf
...
smtpd_tls_cert_file= /etc/letsencrypt/live/domainname/fullchain.pem
smtpd_tls_key_file= /etc/letsencrypt/live/domainname/privkey.pem
# File /etc/dovecot/conf.d/10-ssl.conf
...
ssl = yes
ssl_cert = </etc/letsencrypt/live/domainname/fullchain.pem
ssl_key = </etc/letsencrypt/live/domainname/privkey.pem
is required.