Outlook stopped recognizing LetsEncrypt Cert after renew

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Adding on to my previous post. Instead of the command in that ServerFault thread you might be need to use this command instead:

postmap -F hash:/etc/postfix/sni

See this thread:

Your symptoms match pretty well to that thread and relates to postfix and its SNI config. I see port 465 is still sending the older cert but if I remove the servername (or use a faulty one) with openssl I get the current cert. So, it seems your SNI config with Postfix needs refreshing. It's worth a try. Otherwise Postfix is well beyond my skill set. Good luck.

openssl s_client -connect mail.rdkscorner.eu:465 --noservername | head
---
Certificate chain
 0 s:CN = mail.rdkscorner.eu
   i:C = US, O = Let's Encrypt, CN = R3
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Nov 29 16:54:50 2022 GMT; NotAfter: Feb 27 16:54:49 2023 GMT
or:

openssl s_client -connect mail.rdkscorner.eu:465 --servername FakeName | head
---
Certificate chain
 0 s:CN = mail.rdkscorner.eu
   i:C = US, O = Let's Encrypt, CN = R3
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Nov 29 16:54:50 2022 GMT; NotAfter: Feb 27 16:54:49 2023 GMT

But leaving that off to use your mail.rdkscorner.eu domain gets the old cert.

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