Azure sync question for anyone that can help

I've found the easiest way is to make sure that the user has a primary SMTP address associated with the account under the proxyaddresses attribute in Active Directory. Note that this is not the same thing as the email address field that you set when setting up a user, and it may have to be manually set under Advanced Features in Active Directory Users and Computers. Basically you add an entry to proxyaddresses in the format SMTP:user@domain.com. The upper case SMTP denotes that it is the primary address for the user. You can also have entries with lower case smtp, like smtp:user2@domain.com. The lower case smtp denotes that it's an alias address and not the primary.


Here's an article on various scenarios - LINK

This is where I'd start as well. Called SMTP matching for anyone who wants to look up more about this.