I have googled but with no luck.
on our exchange server it has the primary domain 'companya' and all boxes are setup using this.
Now I have an additional domain that I want to be able to receive and send as from 'companyb'.
I have added the domain tot he accepted domains i.e.;
Exchange -> Organization Configuration ->Hub Transport -> Accepted Domains
and added the relevant accepted email address to the users i.e.;
Receipient Configuration -> Mailbox -> selected box -> email-addresses
So I can receive from email@example.com inside firstname.lastname@example.org's mailbox.
However, when I try to send as email@example.com from outlook i receive the following error
You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission.