I just tested emailing an invoice using the steps outlined on my website and it was sent with no issues.

Can you go here and verify you have the correct information entered?

https://www.fabozzi.net/use-office365-web-mail-in-quickbooks

Part 1 is for MFA users only. If you don't have MS MFA enabled then you can skip to Part 2. My test was done from an MFA enabled account.

Let me know if you have any questions.