I imagine it could cause problems if not implemented correctly. There is a description of what Sender Policy Framework is and what it attempts to do here: http://en.wikipedia.org/wiki/Sender_Policy_Framework
So if the IBM i is configured in DNS with a SPF record, it should work.
But are you talking about the i sending the emails out of the domain directly or relaying through your external mail server? Can you Pseudo diagram the mail flow to clarify?