I'm a little unclear here. I already have domain.onmicrosoft.com in O365 but haven't setup dirsyc yet. When we setup O365 I verified my domain.
I may have been over-reading into things but was trying to follow technet.microsoft.com/.../jj151831 (under REview UPN requirements) where it said to make sure the username is publicly routable. Since users internal UPN is username@domain.internal and not publicly routable, I was wondering if I need to add domain.com as a UPN suffix.