Hello,

I am running a self-hosted email server with Let's Encrypt certificates.
The certificates are valid and properly configured:


- Certificate: Let's Encrypt (issued by R11/R12)
- Valid until: December 2026
When using Thunderbird I get certificate warning

However, Kaspersky products (Internet Security / Endpoint Security)
show certificate warnings when connecting to my mail server:

Error displayed: "Certificate is not trusted"
or: "Unable to verify certificate chain"

Technical details:

- Port: 995 (POP3 SSL) / 993 (IMAP SSL) / 587 (SMTP STARTTLS)
- Certificate chain:
- ISRG Root X1
- Let's Encrypt R11


Questions:
1. Does Kaspersky support Let's Encrypt root CA (ISRG Root X1)?
2. Is there a known issue with newer Let's Encrypt intermediate
certificates (R11/R12)?
3. Are there any workarounds for this issue?

The certificate works perfectly with:

The same goes for website visitis.

I m using Kaspersky Free 21.20.8.505

Only Kaspersky shows the warning.

Any help would be appreciated.

Best regards

Edited September 29Sep 29 by RAS-OS

Welcome to Kaspersky Community.

Hum 🤔 Your KFree 21.20 is a bit obsolete, current 21.26...