Unable to send or receive email for a week

Since the end of last week I now receive the following message (or variations on it):

Message could not be sent. Error message from e-mail transport server: 450 - 450 4.1.8 Sender address rejected: Domain not found

Also, nobody has been able to send me emails since then either. They all receive notification of a delayed delivery.

There has been no change in the custom domain and it is still paid up and valid