Thank you for posting here in Community, case450.
I appreciate you letting me know that you've tried running some tests in sending your invoices.
This error could be caused by different things. One potential cause is an application installed on your computer, such as an anti-virus or security program, that is hindering the ports that QuickBooks needs in order to send email through these web mail applications.
I recommend checking your ports, the following link contains information on this: Set up your email service in QuickBooks Desktop
Have you tried sending the invoices using Web Mail? Let me know as well if you're getting an error message.
You may
wish to consider contacting the Phone Support Team
as they can do a screen share and look over your settings and troubleshoot
through a screen share.
Let me know how this works. You can do so by
replying down below. Thanks!
