Hello Jono, We have spoken with the senior team who have advised that part of the adding Quickbooks to the safe sender process is to configure the server to accept Quickbooks Online mail server host names and IP addresses and if that has not worked the next step would be the server in this case Microsoft 365 confirming there are no issues/blocks at their end. Nigel the customer support agent you're dealing with will be in touch with you about the information Microsoft are requesting and the links they sent.