Here are some links to articles that may or may not help
- Additional SSL certificates for IMAP and SMTP
- How to get iOS email to accept cert renewals for IMAP and SMTP servers?
- Mail Certificate smtp/imap problem
- Using Let's Encrypt Certificates for Mail and other Apps [ClearOS Documentation]
- Using Let’s Encrypt TLS Certificates for SMTP, IMAP, and HTTP
- How to secure Postfix using Let’s Encrypt - UpCloud
- Install FREE Let's Encrypt certificate in Exchange Server - ALI TAJRAN