Delivery Status Notification (Failure)

I’ve been writing to this person in Italy for months and months. For about a month now he can e-mail me at my work and when I respond I receive the following message:

Action: failed
Status: 5.0.0 (permanent failure)
Diagnostic-Code: smtp; 5.1.0 – Unknown address error 550-‘SPF MAIL FROM check failed (PermError)’ (delivery attempts: 0)
Reporting-MTA: dns; vmx4.rbc.com

Has anyone had this problem and do you know what this error means? I sent this message to my support group about 2 weeks a go and they are still investigating! I can’t believe it takes this long? Would someone please help!!
thanks D