In order to figure out how many days your letsencrypt certificates have left, you could check an online services such as:
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
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