Emails bouncing - Diagnostic-Code: rejected

We are receiving the above message when trying to send invoices to existing customers, this has only started happening in the last week but as these customers are Plan Managers who manage multiple clients NDIS plans, there are lots of invoices that have been rejected.

Down South Therapy <bounce@apps.myob.com>

Emails affected

accounts@myplanmanager.com.au

invoices@providerchoice.com.au

Please advise when this issue will be resolved.

Kind regards