I have a couple of customers that I have a delivery issue when emailing an invoice, even though it used to send and email addresses are correct.

It's nice to see you posting here in the Community forum, walkerscaffold.

Let me assist you in resolving the delivery issue you are facing when sending an invoice through QuickBooks Online (QBO).

Customers occasionally don't receive your invoices, sales receipts, estimates, statements, or reports by email for several reasons. This can be due to server maintenance, unexpected downtime, or network disruptions. To get this resolve, it’s advisable to wait and retry sending the email after some time.

If the problem persists, we can start resetting your email address, especially if you recently imported QuickBooks Desktop data into QuickBooks Online (QBO). To do this, here's how:

1. Go to the Gear icon at the top.
2. Choose Account and settings.
3. Go to the Company section.
4. Select Edit ✎ in Contact info.
5. Review each email address and make changes as needed.
6. Select Save, then Done.

After this, resend the invoices. If the error persists, continue to Solution 2 in this article: 3 solutions for when customers aren't receiving your emails.

Additionally, you can manually download the invoice and send it to your customer by attaching it to an email. Here's how:

1. Navigate to Sales, then choose the Invoice tab.
2. Next, open an invoice you want to send. 
3. Then, select the Print or Preview button on the bottom part of the screen. 
4. Click the Download option. 

For future reference, check out this article to learn how to receive invoice payment in QuickBooks Online. 

If there's anything else you need help with in managing your invoices, please let me know by commenting below. I'm always here to answer any questions you may have. Take care, walkerscaffold.