I have an Office 365 tenant with a few Office 365 licenses and their corresponding users and regular email addresses.
I am loosely related to the company and I want to create an email address luis @company.com that will just forward everything to my actual company email address without consuming an extra Office 365 license here (I already pay for it on the other commpany).
What I have done so far:
Created a shared mailbox on the Office 365 admin portal, with no member users, but turning on the Email forwarding
Created a Mail Contact in Exchange Admin Portal with an external email address, and had the above shared mailbox redirect to that mail contact (set in the mail flow part of the shared mailbox in Exchange Admin Portal)
Created a Mail User with an external email address (this has the local email address too)
None of those seem to work when sending email from an external email address.
However I did get Exchange migration notifications when I asked Exchanged to notify the shared mailbox (item 1 above).
You can do this by 1) creating a mail contact or mail user representing yourself in the forwarding Exchange organization (e.g., firstname.lastname@example.org); and then 2) creating a mailflow rule that redirects messages sent to the mail contact or mail user to an external address (e.g., email@example.com).
Keep in mind though that forwarded messages will generally fail the SPF check. (I was under the impression that SRS rewriting is being developed or rolled out to Exchange Online but in my tests it wasn’t used.)