Most invoices I email from QB are giving me an error message to say it is undeliverable or Delivery issue, yet checking with some customers, they have received it. Why?

Hi userreimers78,

Thanks for posting in the Community! If multiple email addresses are setup an any one fails delivery, this undeliverable error message will appear. Let's check your settings to see if an email address has setup there.

1. Go to Gear icon and select Account and Settings.
2. Select Sales.
3. Select Messages. 
4. Check if the email address in CC and BCC are correct/valid. 

Once done, check the customer's profile for multiple email addresses. The extra email addresses may be what is causing the undeliverable messages. 

You can also check this article for your future reference: 3 solutions for you when customers aren't receiving your emails.

Feel free to drop a comment below if you have any other questions. I'll be around to help.