How to add an alias from a different domain to office365 so a user can change the 'from' to match

Ben Cameron
Ben Cameron used Ask the Experts™
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I have a client with an office365 account with a single e-mail address 'user@hisdomain.com'.  This user does outside sales work for other companies that have their own e-mail systems and domain names, and they have created e-mail addresses for my client in their domains 'user@theirdomain1.com and user@theirdomain2.com'.
My client uses office365 OWA, Outlook and Outlook for Mobile to access email
We currently have forwarded 'user@theirdomain1.com to user@hisdomain.com'.  When my client receives an e-mail from 'user@theirdomain1.com' they want to be able to reply to the e-mail and change the 'from' address to 'user@theirdomain1.com'
How can I set this up in office 365 so they are able to change the 'from' address to the proper Domain for the reply to keep e-mail continuity?
Thank you
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Most Valuable Expert 2015
Distinguished Expert 2018
Commented:
You cant, O365 will only allow you to use domains you own and can verify into the tenant as email aliases. You can however add his "other company" email as additional account in Outlook, which will allow him to pick up the address when composing messages. For OWA, the same functionality can be achieved via the "connected accounts" feature: https://support.office.com/en-us/article/connect-email-accounts-in-outlook-on-the-web-office-365-d7012ff0-924f-4f78-8aca-c3912d886c4d

Author

Commented:
will the connected accounts pull in real time, or does the OWA need to be opened? I assume nothing these days:)

Author

Commented:
also, can you elaborate on "You can however add his "other company" email as additional account in Outlook, which will allow him to pick up the address when composing messages." .  Simple rundown/article if you have something for me to reference.  Thanks!
Most Valuable Expert 2015
Distinguished Expert 2018

Commented:
Not in real time, it's once every 30 mins or so. For the second question I simply meant to add the other company's mailbox in Outlook.

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Commented:
Thank you!

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