QBO not sending emails.

I appreciate you for coming back to the thread and providing an error when sending email sales entries in QuickBooks Online. I've got steps to ensure you can get this and can send invoices and estimates efficiently.

One of the troubleshooting steps to fix the error is to check your company and customer-facing email address fields. Then, ensure they are complete, have the correct format, and do not have prefixes. If you haven't performed this yet, you can do so by following the steps below.

1. Click the Gear icon ⚙, then select Account and settings.
2. Choose the Company tab.
3. Select Contact info, then check your Company email and Customer-facing email. Use these examples as a reference on what format to use:

• Correct: mycompany@mycompany.com
• Incorrect: email:mycompany@mycompany.com

Furthermore, you can also check your location’s email address you used on the sales form. 

Here's how:

1. Click the Gear icon ⚙.
2. Choose All lists and Locations.
3. Find the location you used on the invoice.
4. From the Action column, select the dropdown, then Edit.
5. Check your email address in the field “This location has a different email address for communicating with customers” and make changes if needed.
6. Click Save.
7. Try to resend your sales form.

If the problem persists, you can refer to this article for further steps to take to resolve the error: Fix the Delivery Server Down error in QuickBooks Online. 

In case you need steps on how to make any transaction a base template for a recurring transaction, you can read this artilcle: Create recurring transactions in QuickBooks Online.

Inform me if you have any further concerns about handling estimates and invoices in QuickBooks. I will be available to offer valid suggestions for you, NickNath. Have a great day!