If your Dovecot just started telling your that the Letsencrypt R3 certificate expired, make sure you are using the fullchain certificate in your configuration file.
Make sure you have the following in your /etc/dovecot/conf.d/10-ssl.conf
ssl_cert = /etc/letsencrypt/live/mail.foo.com/fullchain.pem
ssl_key = /etc/letsencrypt/live/mail.foo.com/privkey.pem
And restart dovecot afterwards.