The problem with all of this is that Quickbooks is using legacy authentication (Authenticated SMTP) via a Username and Password.  This is being removed from the O365 Platform.  THye have been warning about this for 2+ years now.

You can still do Authenticated SMTP but has to be done via Modern Authentication. The way you do this is via an Enterprise Application in Azure using the Graph API and correct permissions.  This will then allow for the MFA (As it's modern auth aware NOT legacy auth aware - which is why it doesn't work).

For organizations that also use Conditional Access Policies to block legacy auth - they to will be blocked and have problems.  The Modern Auth approach will work with both Conditional Access Policies and MFA.

Quickbooks - you need to give your users an option in sending email via the Microsoft Graph API (via Enterprise application) and API permissions consented to (on behalf of organization).

Recommendations in this thread actually make O365 platform less secure!

If you don't do this soon you are going to start having some really angry customers!