Spring mockito bean still does nothing, despite doAnswer setting

I'm trying to stub an injected dependency upon its void method:

@Autowired
private FeedbackService service;
@MockBean
private MailSender sender;

@Test
public void testMonitor() {
    // mocking MailSender#sendMessage
    Mockito.doAnswer(invocation -> {
        log.info("SUBJECT: {}", invocation.getArgument(0, String.class));
        log.info("CONTENT: {}", invocation.getArgument(1, String.class));
        for (String dest : (String[]) invocation.getArgument(2)) {
            log.info("DEST: {}", dest);
        }
        return null;    // Void anyway
    }).when(sender).sendMessage(anyString(), anyString(), any(String[].class)); //FIXME still doNothing
    // invoking the service which calls MailSender#sendMessage
    service.monitor();
}

But logging and debugging shows that no interception occurs at runtime. Does anybody know what I'm doing wrong, please ?