URGENT !!! receive mail

URGENT !!! receive mail

I'm having trouble receiving emails from many customers, trying to invoke emails but receive back type errors like this:
---
----- Messaggio Originale -----
Da: "Mail Delivery Subsystem" < MAILER-DAEMON@ghassisi.eu>
Data: 29/08/2017 09:37
Oggetto: Returned mail: response error

Il messaggio originale è stato ricevuto in data: Tue, 29 Aug 2017 09:37:22 +0200

 ----- I seguenti indirizzi hanno errori fatali permanenti -----
< info@viva-voce.it>

 ----- Transcript of session follows -----
.. while talking to  mx.zoho.com
>>> RCPT To:< info@viva-voce.it>
<<< 550 5.7.23 Sender's SPF Policy Failure

Reporting-MTA: DNS;  posta.solinformatica.it
Arrival-Date: Tue, 29 Aug 2017 09:37:22 +0200

Original-Recipient: RFC822; < info@viva-voce.it>
Final-Recipient: RFC822; < info@viva-voce.it>
Action: failed
Remote-MTA: DNS;  mx.zoho.com
Last-Attempt-Date: Tue, 29 Aug 2017 09:37:25 +0200

---

What does it mean please?
What should I do?
It's really URGENT PLEASE.