Emails using office 365 are not being sent

Let me shed some light on the undelivered invoices, accounts-stac.

Some customers have reported that they encountered an error when sending the sales form. I can see that this has been inconvenient for you. Rest assured, our engineers are committed to resolving the issue in a timely and effective manner.

That said, I recommend contacting our QuickBooks Online (QBO) Care Team. This is to ensure you can sign up for the list of affected users. You'll be notified via email once there's progress in the investigation. 

1. Hover your mouse to the Help menu in the upper right to see more options.
2. Go to the Assistant tab and choose a suggested option or type a question or topic in the field box.
3. Select Contact Us and type a question.
4. Press the Continue button and choose Start a chat to initiate a conversation.

You can browse through this article for more details about the support hours: Contact QuickBooks Online.

In addition, I am sharing this link that contains troubleshooting tips for undelivered emails: 3 solutions for you when customers aren't receiving your emails

Thank you for your understanding and patience as we work towards resolving this matter. Feel free to visit the Community again if you have additional questions about invoices and other sales-related concerns. I'm always ready to assist you further.