The answer to your question might be answered by the FAQ:
Does Let’s Encrypt issue certificates for anything other than SSL/TLS for websites?
Let’s Encrypt certificates are standard Domain Validation certificates, so you can use them for any server that uses a domain name, like web servers, mail servers, FTP servers, and many more.
Email encryption and code signing require a different type of certificate that Let’s Encrypt does not issue.
Does the version of exim4 that's available under Debian 11 support SMTPS? If so, does it use standard x.509 certificates? Consult its documentation to see. And if the answer to both questions is "yes," then you can use a cert from Let's Encrypt for that purpose.
It really isn't a correction, though perhaps the FAQ could be more clear. The first paragraph quoted above says that LE certs can be used for any server that uses a domain name including mail servers--that's the relevant part of the answer for your question. The "email encryption" mentioned in the second paragraph is talking about encrypting emails as the end user, specifically using S/MIME. That, like code signing, requires a different type of cert.