I was having the same issue after enabling MFA on our Office accounts. After creating an app password in office, it still wasn't working. At first I couldn't create an app password at all, and the only option was to add authenticator app. In order to see the option to add an app password, you need to make sure that MFA is set to enforced. Admin Center > Users > Active Users > Multi Factor Authentication. This will bring up a window that shows all users and on the far right, under "Multi-Factor Auth Status" it will either say: Enabled or Enforced. Make sure it's set to Enforced.

If you don't know how to add an app password this is how:

Login to office > Click your user bubble in top right > View Account > Security Info > Add sign -in method > Choose App password > Name it Quickbooks > and then save that password somewhere safe. They will only show it to you once. You will use this app password as your email password when sending email out of QB.

After i set up the app password it still wasn't working, and that was due to SMTP Authentication not being set. There are couple ways to check this. 

First Way

Admin Center > Users > Click User > Pop up will come up on right side of Screen > Click Mail tab at Top > Manage Email Apps > Make sure Authenticated SMTP is checked

Second Way

Even after checking Authenticated SMTP in mail settings, my email still wasn't working. When MFA was initially setup in our account, it asks if you want to use the security defaults. If you select yes, then there is a block of settings in Org settings called Modern Authentication, that you will not be able to change if Azure's default security settings are being used. To access these settings you need to navigate to aad.portal.azure.com

Click Azure Active Directory on the left pane > Under Manage > Click Properties> Click "Manage Security Defaults" on very bottom. Should be in small blue font > Window will pop up in left pane. Click No under "Enable Security Defaults", and fill out the reason why you're changing > Hit save.

Now go back to Office Admin Center > Clock Settings on left > Org Settings > Modern Authentication > Make sure "Authenticated SMTP" is set. 

Should be all set now. Make sure you create an app password called Quickbooks, and when you go to send an email through quickbooks use that app password instead of your actual account password. 

Email setting should be the following in QB:

Email ID: Your Email

Email Provider: Others

Server Name: smtp.office365.com

Port: 587

Enable:  SSL/TLS

Hope that helps anyone who stumbles upon this. This is what worked for me using Outlook and Quickbooks Desktop 2021