ServerName not used?

[mdosch]

According to [1] ServerName should be used to verify the cert:

// ServerName is used to verify the hostname on the returned
// certificates unless InsecureSkipVerify is given. It is also included
// in the client's handshake to support virtual hosting unless it is
// an IP address.
ServerName string

But when I connect to e.g. the server xmpp.example.org:443 with the user user@example.org and set ServerName like the following example I get this error:

2020/04/05 21:04:35 x509: certificate is valid for example.org, [other domains but not xmpp.example.org] not xmpp.example.org

        var tlsConfig tls.Config    
        tlsConfig.ServerName = strings.Split(user, "@")[1]
        tlsConfig.NextProtos = append(tlsConfig.NextProtos, "xmpp-client") 

        // Set XMPP connection options.    
        options := xmpp.Options{        
                Host:      server,    
                User:      user,    
                [...]
                TLSConfig: &tlsConfig,    
        }   

As I understand the explanation from crypto/tls it should not fail as it should check for example.org and not xmpp.example.org although connecting to xmpp.example.org that's why I suspect the library is ignoring my setting of ServerName.

[1] https://golang.org/pkg/crypto/tls/#Config