When setting up a Caldav connection to said server, emclient fails. For debugging purposes, I’ve set up a testaccount over plain http and checked the communicaton with wireshark:
Hypertext Transfer Protocol
OPTIONS / HTTP/1.1\r\n
Host: 192.168.100.241:5232\r\n
User-Agent: eMClient/10.4.5326.0\r\n
Accept-Encoding: gzip, deflate, br\r\n
\r\n
Hypertext Transfer Protocol
HTTP/1.1 401 Unauthorized\r\n
WWW-Authenticate: basic realm=“null”\r\n
Content-Length: 0\r\n
Server: Jetty(9.4.22.v20191022)\r\n
\r\n
Hypertext Transfer Protocol
OPTIONS / HTTP/1.1\r\n
Host: 192.168.100.241:5232\r\n
User-Agent: eMClient/10.4.5326.0\r\n
Accept-Encoding: gzip, deflate, br\r\n
Authorization: Basic QWRtaW46VGVzdGFkbWlu\r\n
\r\n
Hypertext Transfer Protocol
HTTP/1.1 200 OK\r\n
Date: Tue Apr 28 00:00:00 CEST 2026\r\n
HTTP/1.1: 200 OK\r\n
Server: advolux-caldav-server\r\n
Content-Type: text/plain; charset=“utf-8”\r\n
Allow: DELETE, GET, HEAD, MKCALENDAR, MKCOL, MOVE, OPTIONS, PROPFIND, PROPPATCH, PUT, REPORT\r\n
DAV: 1, 2, 3, calendar-access, addressbook\r\n
Content-Length: 0\r\n
\r\n
After that response, emclient fails with log:
eM Client 10.4.5326+97a2e75a43 (Windows)
12:02:58 Online-Status: geändert zu online bis zum NetworkAvailability
12:02:58 Admin CalDAV / CardDAV Kontoeigenschaften werden synchronisiert: Für Host http://192.168.100.241:5232/
12:02:58 Admin [CalDAV / CardDAV] MailClient.Accounts.ConnectionException: Received an invalid header name: ‘HTTP/1.1’.
—> System.Net.Http.HttpRequestException: Received an invalid header name: ‘HTTP/1.1’.
at System.Net.Http.HttpConnection.g__ThrowForInvalidHeaderName|67_1(ReadOnlySpan1 name) at System.Net.Http.HttpConnection.AddResponseHeader(ReadOnlySpan1 name, ReadOnlySpan1 value, HttpResponseMessage response, Boolean isFromTrailer) at System.Net.Http.HttpConnection.ParseHeadersCore(Span1 buffer, HttpResponseMessage response, Boolean isFromTrailer)
at System.Net.Http.HttpConnection.ParseHeaders(HttpResponseMessage response, Boolean isFromTrailer)
at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.AuthenticationHelper.SendWithNtAuthAsync(HttpRequestMessage request, Uri authUri, Boolean async, ICredentials credentials, Boolean isProxyAuth, HttpConnection connection, HttpConnectionPool connectionPool, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
at System.Net.Http.AuthenticationHelper.SendWithAuthAsync(HttpRequestMessage request, Uri authUri, Boolean async, ICredentials credentials, Boolean preAuthenticate, Boolean isProxyAuth, Boolean doRequestAuth, HttpConnectionPool pool, CancellationToken cancellationToken)
at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at System.Net.Http.DecompressionHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
at MailClient.Protocols.InteractionController.HttpClientCertificationValidationMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at MailClient.Protocols.Common.HttpClientHandlers.HttpClientBackoffHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpClient.g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
at MailClient.Protocols.CalDav.ProtocolCommands.Connector.RunCommand(CalDavAccount account, ICommand command, CancellationToken cancellationToken, Boolean checkBootstrap)
— End of inner exception stack trace —
at MailClient.Protocols.CalDav.ProtocolCommands.Connector.RunCommand(CalDavAccount account, ICommand command, CancellationToken cancellationToken, Boolean checkBootstrap)
at MailClient.Protocols.CalDav.FolderSynchronizer.UpdateOptions(Uri baseUris, Dictionary`2 namespaces, CancellationToken cancellationToken)
at MailClient.Protocols.CalDav.FolderSynchronizer.Bootstrap(WorkerStatus status, CancellationToken cancellationToken)
at MailClient.Protocols.CalDav.CalDavGenericCommand.EnsureBootstrap(WorkerStatus status)
at MailClient.Commands.EnsureBootstrapCommand.Execute(WorkerStatus status)
at MailClient.Commands.Command.Process(WorkerStatus status)
AccountId = a42946d0-cbb0-40c9-9449-3541537d7b59
AccountName = Admin
RunInPlace = False
GUIStatus_Exception_Reported = True
The Header name “HTTP/1.1” should be fine though?