Thanks for posting to the Community, @rob-wilde.

We have lots of includes to allow it to work without reporting too many lookups when sending invoices, sales receipts, estimates, statements, etc.

Aside from include:intuit.com, you can try using different server hostnames that on the list through this article: 3 solutions for when customers aren't receiving your emails. There you can see the possible solutions if your customers were unable to receive statements.

Additionally, you can reach our Intuit Developer site if you have more questions about SPF.

Feel free to place a comment below if you have other concerns about QuickBooks. I'll be around to help. Stay safe and well!