I have set up Zoho Mail to work with my own domain, and it seems to work with Gmail and Yahoo addresses just fine, but I keep getting this error when sending to this one email. (I've censored personal details in the error message below)

Could you help me figure out what is wrong? I have added MX and SPF records for my domain so I don't understand why I cannot send this email.

Thanks for any help in advance!

This message was created automatically by mail delivery software. 

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. 

***@***.com INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.1.1 <***@***.com> Recipient not found. <http://x.co/irbounce> 

Received:from mail.zoho.com by mx.zohomail.com 

with SMTP id 1524581235282565.478186110329; Tue, 24 Apr 2018 07:47:15 -0700 (PDT) 

Message-ID:<162f81f793d.bb92b84d12242.1801477646981407078@***.me>