We (companya.com) have a partner organization that we work with (companyb.com). Some of CompanyB's employees have AD accounts here with Exchange mailboxes. They then requested that their mail redirect back to their home (CompanyB) mailbox, so a contact was created for them, and the exchange settings for their account were then changed to forward mail to their contact rather than keep it locally. Not the most graceful perhaps, but should work.
The problem I am having is some people from CompanyB want to send email to one of our distribution lists (firstname.lastname@example.org). This list contains some members of our domain that forward their mail back to companyB. When sending to these lists, the sender gets an SMTP 5.7.1 error indicating that relay is denied. Makes sense since companyb.com is an external domain that my mail server should not relay to by default.
To try and work around this, I created an SMTP Connector under 'Routing Groups' that lists companyb.com as the namespace, checked the box for 'allow relaying' and assigned a bridgehead server to the connection. For some reason I am still unable to send mail from outside the company to this dist list, still getting the relaying error.
How can I setup my exch environment to allow mail relaying to just this one domain?