Hey guys,

We have a Exchange 2013 server with both mailbox and client role installed.

This is receiving mail from different domains, works like a charm.

Now our client would like to be able to send on behalf from a domain we do not host, and does not point to our server. Is this even possi

Possible? yes. Ethical? No. What the user wants to do is basically "spoofing", which might not even work at all in the domain in question has setup SPF records in DNS. Then the message could be marked as spam or blocked entirely and potentially blacklist your sending IPs. Very bad idea. 

That corresponds with my findings on the internet. Thank you for the info.