Thank you Bourne, that is very helpful information to get me started.
We do have an in house Exchange infrastructure with company.com as the email domain, so there shouldn't be an issue there.
One other question.
Let's say I add the UPN suffix company.com, and change the log in for user1 from user1@company.local to user1@comany.com, when that user logs into their domain joined computer (computer1.company.local), nothing should change correct? We use the standard userid & password to login to domain joined machines. So basically, my concern is that something will change with regards to authentication, even though it shouldn't because the SID is exactly the same.