I have been experiencing the same issue with QuickBooks Online throughout this year.  It looks like Intuit does not have an SPF record for their sub-domain notification.intuit.com which they use to email out my QuickBooks Online invoices.  All my customers that have spam filtering with SPF record checking cannot receive my invoices due to this problem.

Can anyone at Intuit solve this problem???  I call tech support and they do not understand.