I understand the convenience of working your invoice emails without any issues, lostdutchman. Let me help you configure your server to accept QuickBooks Online mail server hostnames and IP addresses.

Many email servers won't accept messages from IP addresses that don't have reverse DNS records that correspond to the domain name that the server claims to represent. This is an anti-spam strategy that stops spammers from posing as well-known domains.

Since this requires technicalities on your end, it is best to work with an IT professional to help configure and accept QuickBooks Online mail server hostnames and IP addresses to allow invoice emails to transit via your mail servers. Thus, you can follow Solution 3: Configure server to accept QuickBooks Online mail server hostnames and IP addresses outlined in this article: Troubleshoot if customers aren't receiving emails from QuickBooks Online.

Moreover, I've included this reference to help you add specific customizations and achieve professional-looking sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

We're here to back you up in this forum if you require additional assistance managing sales transactions. Please let us know in the comments below. Stay safe!