Fun fact, if you sync users who have email aliases on office 365, but not the local AD profile those aliases get deleted from office 365.
The solution is to add the appropriate aliases to the users AD profile using ADSI edit. The values go into a field called “proxyAddresses”. Each address should start with “smtp:” if it is a secondary address (aka: alias), but if it is the users primary address it has to start with “SMTP:”
You need to add the users primary address to that field in order to prevent office 365 from changing the users primary address to the *.onmicrosoft.com variant. Continue reading