CalDAV SSL Handshake failure on macOS with ECC certificate CalDAV server: SOGo

Calendar
1

Hi everyone,

I’m experiencing an issue with EM Client on macOS where CalDAV synchronization fails with an SSL handshake error. I’ve done extensive troubleshooting and I’m fairly confident this is a bug specific to EM Client on macOS.

Environment:

  • EM Client 10.4.4867 (MAC)
  • macOS 26.3.1 (25D2128)
  • CalDAV server: SOGo
  • Server certificate: ZeroSSL ECC Domain Secure Site CA (TLSv1.3)

The problem: Every time EM Client tries to connect to our CalDAV server, it fails immediately with an SSL handshake error. The connection never gets established.

What I have already verified:

  • The macOS native Calendar app connects to the same server successfully :white_check_mark:
  • EM Client on iOS connects to the same server successfully :white_check_mark:
  • curl on macOS connects successfully using TLSv1.3 :white_check_mark:
  • The server certificate chain is complete and valid :white_check_mark:
  • The ZeroSSL intermediate certificate is installed in the macOS keychain :white_check_mark:

This clearly shows the issue is not on the server side, but specific to EM Client on macOS. The pattern points to a problem with the .NET HttpClient implementation on macOS when dealing with ECC-based certificates.

Error from EM Client logs:

Interop+AppleCrypto+SslException: Fehler beim Handshake
System.Security.Authentication.AuthenticationException: net_auth_SSPI
System.Net.Http.HttpRequestException: net_http_ssl_connection_failed
MailClient.Accounts.ConnectionException: net_http_ssl_connection_failed

Has anyone else experienced this issue or found a workaround? Any help is appreciated.

2

Suggest if you have a current active paid Pro, Personal or Business version to go to the VIP Support page and login and lodge a support ticket who can investigate that for you.