Let's perform some troubleshooting steps to send the emails successfully in QuickBooks Self-Employed (QBSE), davidbeeney. 

When an email is sent from your QuickBooks company, the email goes out from our server. As your recipient receives it, it will look like it came from your email provider (ex. @Yahoo.com, @gmail.com). When the email bounces, the recipient's email service sends a failure or delivery error notification to your email service. If the delivery failure error is sent to us by your email service provider, a notification is added to the transaction stating the email delivery failed.

To resolve this one, just Reset your email address. Then, ask your customers to check their junk mail and spam folders. If the email is still not found, clear and then re-enter your email address. Then, try sending yourself a test transaction again. If you still don't receive the email, try using a different email address that has a different domain (for instance, if you were using a Gmail email address, try a Hotmail or Yahoo address instead.

If the same thing happens, we may need to configure the server to accept QuickBooks mail server hostnames and IP addresses. Just follow the troubleshooting solution in Step 3 in this article: 3 solutions for when customers aren't receiving your email. Thought the article is for QuickBooks Online, it still works for QBSE version. We can also contact your IT expert to have the QuickBooks mail server hostnames and IP addresses are added to your outgoing mail servers.

Feel free to visit our Transactions page for more insights about managing invoices, payments and transactions.

I'd like to know how you get on after trying the steps, as I want to ensure this is resolved for you. Please reply to this post and I'll get back to you. Take care always.