Undeliverable Invoices/Statements

Hi 

Over the past couple of days we have been getting multiple 'undeliverable' messages when trying to send invoices/statements. The diagnostic code just says rejected. The strange thing is when we have sent invoices/statements previously (the past couple of weeks) to the email addresses we are getting these messages on now,  and they have gone through with no issues. Now all of a sudden we are getting undeliverable messages on these same email addresses that have worked previously. How do we fix this? Nothing has changed within the last few days, email addresses are still the same and are correct. Can you please advise?

Thank you