We have a reported issue about Invoice & Purchase Orders Delivery Error Message: Delivery Server Down (INV-28290). Rest assured our Product Engineers are working on releasing a permanent resolution about this ongoing concern.
Meanwhile, if you’re not getting the same error prompt, we can perform some troubleshooting steps to isolate this for possible browser-related:
Sign in to your QuickBooks Online account using an incognito or private browser:
Google Chrome, press CTRL + Shift + N
Mozilla Firefox: press CTRL + Shift + P
Internet Explorer: Press CTRL + Shift + P
Safari: press Command + Shift + N
If you’re able to send your sales forms, you may want to clear the cache of your regular web browsers.
If the issue persists, you can try to use another web browser.
You can read through these articles for more insights about sending sales forms thru QuickBooks: