What does "550 Denied by policy" mean for Email Suspended Cause

[Anonymous]

We have a prospect in our Marketo database who is not receiving Marketo email from us. The Marketo lead record has Email Suspended = True and Email Suspended Cause = 550 Denied by policy. 

What can we do to resove this?

[Anonymous]

Are SPF and DKIM set up or, at least. SPF?

The 550 error code can be a symptom is missing SPF.
The receiving server is probably enforcing authentication.