Hi Paul,

We've been through all the troubleshooting (I've spent 40+ hours on this so far this week) as has the IT Consultant who has also received the same confirmations from QuickBooks's technical team. Unfortunately we have absolutely no control at our end as this is the  full extent of the email settings:

Contact info
Company email
Where QuickBooks can contact you.
example<@>example.com

Customer-facing email
Where your customers can contact you. Shown on sales forms.
accounts<@>example.com

They have confirmed that the customer facing example (accounts<@>example.com) is the email address being spoofed to send invoices etc. They could all of course be lying but several operators going into extensive detail about how their spoofing works to hide the fact that their intuit email servers are temporarily not working correctly seems a little more than bizarre.