Das klingt dann nach einem Fall für den "hey" support.
openssl s_client -showcerts -connect smtpauths.bluewin.ch:465
---
Server certificate
subject=C = CH, ST = Bern, L = Worblaufen, O = Swisscom (Schweiz) AG, OU = IT, CN = smtpauths.bluewin.ch
issuer=C = CH, O = SwissSign AG, CN = SwissSign Server Gold CA 2014 - G22
---
No client certificate CA names sent
---
SSL handshake has read 4373 bytes and written 319 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol: TLSv1.2
Cipher: 0000
Session-ID: xxx
Session-ID-ctx:
Master-Key:
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1625048532
Timeout: 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: no
---