Problem: A company split and they were using Office 365 for their email provider. Company A kept the domains. Company A kept all the existing user accounts and email addresses. Company B created a new Office 365 account with their own domain and new email addresses. We needed to be able to forward Company B’s emails to their new addresses, but Company A didn’t want to pay for their Office 365 licenses. Here is the resolution we came across. It worked quite well.

Resolution: I logged into Company A’s portal. I deleted the Company B’s user accounts that were no longer being used.

I then went into the Exchange admin center and clicked on Contacts.

                                                       I then created a new mail user.

I filled in the credentials for the new mail users. Note, make sure you keep the alias and the user ID the same (ex. Alias: test.user and then user ID: would be test.user@domain.com). In the external email address field I put in their new email address. I clicked save 

and then sent an email to the alias. It wasn’t long and an email showed up in my inbox. The forwarder worked without a hitch!

This was posted by techspeeder.

Source: Office 365 Forum