Customer is using Office 365 as backend and uses several domains: company1.com, company2.com, etc.
Each user has one mailbox and assigns multiple aliases/addresses to that mailbox: email@example.com, firstname.lastname@example.org, etc. Any mail sent to any of the aliases ends up in the user's mailbox, fine.
On the client side, users are using Macs with the built-in Mac Mail application. The users must be able to select the alias they want to use as sender's email address. How can this be achieved ?
Things tried that did not work: adding the aliases into the account preference (you can then select an address to send from, however emails always appear to come from the default address), using shared mailboxes for the additionnal email adresses and access them via IMAP (sending works but emails are downloaded multiple times, a sending only IMAP account would be nice)
Any help would be appreciated.