3/4 weeks us a long time! It's happened to me a couple of times. I got a peak of 'undelivereds' after 2-3 days and it was all over in a week.

The undelivered messages are not affecting anyone else, perhaps you can filter them out. It's only if your own messages get bounced that you'll be forced to do something. Maybe test with a few friends to check if your emails are still getting through and look out for cases where they might not be. Good luck.