What do I do if inbound emails are rejected due to SPF?

If an email is rejected due to SPF (Sender Policy Framework) then it is preferable that the sender corrects their SPF record to add the locations they are sending from or they send from a location that is already specified in their SPF record.

Kitterman is a good website where you can check the current SPF record for a domain and also check a proposed SPF record you plan to add or update for a domain.