I have just created my first invoice to email. I get a message that "Invoice hasn’t been sent. We can’t reach this customer." I tried several email addresses. All fail.

Hi, I received a message telling me that the case had been closed, but it hasn't been solved. Steps 1 and 2 didn't work. My IT support told me that step 3 was not relevant as the email to the customer was not gong through my company outgoing mail server, it was going from Intuit's server. I also tried to set up a gmail account to send the invoices from, but this failed too. So, he believes that adding hostnames to allow lists won't make a difference.