I'm in the process of moving a customer from an on-site Exchange to Office 365, but I've hit a roadblock. One of the mailboxes needs to send from an external address, instead of an address on the domain they own. This works fine with their on-site Exchange, as I can just change their default SMTP address. A POP3 connector fetches mail from the external address, and any mail they send leaves their Exchange server as that address. They essentially have Exchange functionality for their external POP3 address.
Office 365 Exchange won't let me add the external address, as it's not an accepted domain. I can't add it as an accepted domain because I can't verify the domain (the domain is owned by the franchisor, not my customer the franchisee). I've added the external address, via OWA, as a Connected Account. I've changed the Default Reply Address to the external account. I've given it a few days to synchronise, but it still wants to send from their own domain via Outlook and OWA. The best I can do is to manually select the From address in OWA, and that works (except it sends it "On behalf of").
Has anybody come across this hurdle before and been able to solve it?
Some people have claimed that the Connected Account and Default Reply Address in OWA has worked for them, even when sending from Outlook, but it's not the case in my testing. I'd be happy with a result that sends directly from the O365 servers (there's no SPF record on the franchisor's domain), or even if there's a way to send via the franchisor's SMTP server. I'd really like to avoid the "on behalf of" scenario, but it's better than nothing.