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

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
Ben CameronAsked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Vasil Michev (MVP)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

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
Ben CameronAuthor Commented:
will the connected accounts pull in real time, or does the OWA need to be opened? I assume nothing these days:)
Ben CameronAuthor 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!
Vasil Michev (MVP)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.
Ben CameronAuthor Commented:
Thank you!
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Outlook

From novice to tech pro — start learning today.