[Outlook 365] Send as another adress when TO: is not in own organisation

Hello Geoffroy PINSON,

As far as I know, this is not possible by default with Exchange Online.

To do this in exchange 2016, you would have to use the following command - https://docs.microsoft.com/en-us/powershell/module/exchange/mail-flow/New-AddressRewriteEntry?view=exchange-ps

However that command is designed to run on the edge server, which you do not have access too in O365. This blog post talks about how it would function as a whole, but again you can see this is talking about the edge servers which as customers of exchange online, we do not have access too:
https://blogs.technet.microsoft.com/exchange/2017/10/26/why-is-my-address-rewriting-not-working-as-expected/

To my knowledge you would need to either add in some sort of third party, or additional hop in the sending chain. Something like:

1. If the from address is customerservices@domain.fr, forward the email to this server)

2. Accept the mail at that server, do your manipulation and changes (perhaps with the above commands, perhaps with a third party tool), then send it back out.

Sorry there is not something out of the box, but that is where you stand, hope this helps.

Adam