If you want to have a single mailbox on Office 365 and be able to send as aliases of that mailbox, you will need to do some work around as it is not really support by Microsoft, see: https://techcommunity.microsoft.com/t5/Office-365/Sending-email-as-alias-in-office365/td-p/60802 https://techcommunity.microsoft.com/t5/Office-365/Sending-an-email-as-an-alias/td-p/98103 1 – Create Distribution List Create distribution group for the desired email address (ensuring is does not exist as an alias or otherwise in the tenant) Add desired destination mailbox.. Read More