Outlook send mail from different domain (Office365/G-Suite)

Here's my situation. Our main domain is example.com setup on G-Suite. We also have a secondary domain example.org setup on Office365. When someone emails user@example.com, it gets forwarded to user@example.org so they can open the emails in Outlook & use the Exchange features. I'd like for when the user replies to the email the From and/or Reply-to fields be populated with the primary user@example.com email address. I tried typing in user@example.com in the "Send From Other E-mail Address", but when I send an email, I received this error:

This message could not be sent. Try sending the message again later, or contact your network administrator. You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x0004dc-0x000524].

Any suggestions on how I can receive emails from my Office 365 acct., but send through my G-Suite acct.?

Turn off the forwarding. Add both accounts to Outlook.

https://support.office.com/en-us/article/send-email-from-a-different-address-in-outlook-com-ccba89cb-141c-4a36-8c56-6d16a8556d2e