private void checkAuthPlainSupport() throws AuthenticationNotSupportedException {
        final String mechanismsString = smartClient.getExtensions().get("AUTH");
        if (mechanismsString == null) {
            throw new AuthenticationNotSupportedException("Cannot authenticate, because the AUTH extension is "
                    + "not supported by the server. Maybe the server expects "
                    + "TLS first");
        }
        final Set<String> mechanisms = parseMechanismsList(mechanismsString);
        if (!mechanisms.contains("PLAIN")) {
            throw new AuthenticationNotSupportedException("Cannot authenticate, because the PLAIN mechanism is "
                    + "not supported by the server. Maybe the server expects "
                    + "TLS first");
        }
    }
Checks if the server supports this mechanism.

@throws AuthenticationNotSupportedException if the server does not support
                                            this mechanism or authentication
                                            at all.
src/test/java/de/gessnerfl/fakesmtp/smtp/client/PlainAuthenticator.java