Mail MessageGateway is null

I created a mail service class, which should use the CQ Mail Configuration.
However I compared my code to several samples from the Internet my service does not work, because
messageGatewayService.getGateway(...) always returns null. This is my simplified code:

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import org.apache.commons.mail.Email;
import org.apache.commons.mail.HtmlEmail;
import org.apache.commons.mail.SimpleEmail;
import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Reference;
import org.apache.felix.scr.annotations.Service;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.day.cq.mailer.MessageGateway;
import com.day.cq.mailer.MessageGatewayService;

@Component(immediate = true)
@Service(MailService.class)
public class MailServiceImpl implements MailService {

    private static final Logger LOGGER = LoggerFactory.getLogger(MailServiceImpl.class);
    
    @Reference
    private MessageGatewayService messageGatewayService;
    
    @Override
    public void sendMail(String subject, String mailText, List<String> recipients) {
        LOGGER.info("Send mail");
        try {        
            Email email = new SimpleEmail();    
            for (String recipient : recipients) {
                email.addTo(recipient, recipient);
            }
            email.setSubject(subject);
            email.setMsg(mailText);
            
            MessageGateway<Email> messageGateway = messageGatewayService.getGateway(Email.class);
            // --> returns null
            MessageGateway<SimpleEmail> messageGateway1 = messageGatewayService.getGateway(SimpleEmail.class);
            // --> returns null
            MessageGateway<HtmlEmail> messageGateway2 = messageGatewayService.getGateway(HtmlEmail.class);
            // --> returns null
            //MessageGateway messageGateway3 = messageGatewayService.getGateway(com.day.cq.mailer.impl.DefaultMailService.class);
            // --> deactivated in code, because DefaultMailService.class unknown

            if (messageGateway!=null) {
                messageGateway.send((Email) email);
            } else {
                LOGGER.error("Message gateway could not be retrieved.");
            }        
        } catch(Exception ex) {
            LOGGER.error("Error on sending mail.");        
        }            
    }
}
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I tested the params used in my CQ Mail configuration in a different mail application and mails were sent out correctly, so the configuration seems not to be the problem.

When I debug the messageGatewayService class I can see it contains a map for gateway configurations which contains the CQ Mail configuration I defined in OSGI (see attachment).
So the messageGatewayService knows about my CQ Mail config. But it resists to deliver it to me :(

I noticed that the CQ Mail configuration in the gateway map is stored with key 'com.day.cq.mailer.impl.DefaultMailService' so it is the PID of the service.
But all examples I saw get the gateway via
MessageGateway<Email> messageGateway = messageGatewayService
.getGateway(Email.class) or
.getGateway(SimpleEmail.class) or
.getGateway(HtmlEmail.class)
which does not work for me?

Any idea?