In order to figure out how many days your letsencrypt certificates have left, you could check an online services such as:
https://www.ssllabs.com/ssltest/index.html
However, that won’t work for a mail server that’s not also running a web server using the same certificate.
In this case, your best bet is to check the certificate file directly using the openssl command as follows:
openssl x509 -noout -dates -in /etc/letsencrypt/live/smtp.yourmailserverdomainname.com/cert.pem