Hier die Ausgabe von

openssl s_client -CApath /etc/ssl/certs -connect imap.hispeed.ch:993

auf meinem Linux Mint 22.3

depth=0 CN = mail.hispeed.ch
verify error:num=20:unable to get local issuer certificate
verify return:1
depth=0 CN = mail.hispeed.ch
verify error:num=21:unable to verify the first certificate
verify return:1
depth=0 CN = mail.hispeed.ch
verify return:1
---

v:NotBefore: Jul 28 10:14:27 2026 GMT; NotAfter: Feb 12 10:14:27 2027 GMT
---

...

subject=CN = mail.hispeed.ch
issuer=C = BE, O = GlobalSign nv-sa, CN = GlobalSign GCC R46 DV TLS CA 2025
---

SSL handshake has read 2594 bytes and written 443 bytes
Verification error: unable to verify the first certificate
...
Verify return code: 21 (unable to verify the first certificate)

Sieht das gut aus ?
Vermutlich nicht.