DNS Error: NXDomain

DNS Error: NXDomain

I have a couple of people that we are trying to send emails to that keep getting bounced back to us (interestingly one of them is an email with a zoho domain). The email addresses are being entered properly and we can send them emails from Gmail accounts. What's the deal?

ERROR_CODE :512, ERROR_CODE :5.4.4 DNS error:NXDOMAIN. Domain not found :alto.esc7.net 

Reporting-MTA: dns; mx.zoho.com 
Arrival-Date: Wed, 5 Jun 2019 17:54:45 -0500 
Original-Recipient: rfc822;  <redacted> 
Final-Recipient: rfc822; redacted Status: 512 
Action: failed 
Last-Attempt-Date: 5 Jun 2019 22:55:06 GMT 
Diagnostic-Code: redacted, ERROR_CODE :512, ERROR_CODE :5.4.4 DNS error:NXDOMAIN. Domain not found :alto.esc7.net 
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