Why are invoices showing up as "undelivered" when it's the same address that works 95% of time?

Hi there,

Thanks for taking the time to post your concern in the Community. I'll be happy to share more info on this topic.

I appreciate you checking to make sure the email is correctly entered. Based on what you've described, there may be a temporary issue with the receipients email server. I encourage checking with the receipient to make sure they're able to receive emails at the moment. You can also try sending the email again at a later time. 

Its important we check to make sure this isn't a cache related issue. Clearing the cache will allow you to work with a clean slate and fix performance errors within QBO. This article will walk you through the steps of clearing the cache, depending upon which browser you're using: How to clear cache and cookies.  Give this a try and let me know how you make out. I'll be on standby.