I have the same problem sending invoices to someone whose email address is in the .energy domain. I've done all of the troubleshooting suggested by QBO, but they still can't send the emails, even though I can send the emails to the client directly without any problems. QBO can send all of the other invoices, and I'm convinced that it's a problem with specific domains, but QBO support can't seem to get past their canned "check this, then this" troubleshooting scripts.