When you click enforce Azure MFA your users will not be able to Connect to Office 365 With Clients that does not support modern auth.

When they set up Outlook they will see this screen and be stuck there:

1 - autodiscover
Without modern auth enabled on Office 365 and Outlook your users will be stuck here

 

When you have enabled MFA on your Exchange Online Tenant this is what will happen:

2 - modern auth prompt3 - mfa prompt in outlook 2016

 

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