Mail not sending although it states mail sent and its put into sent items

Mail not sending although it states mail sent and its put into sent items

Please advise urgently. I have been trying to send email the whole day via Zoho Mail. Zoho Mail states the email has been sent and is added to the sent items folder but none of the emails are received! I have also suddenly received lost of emails like the below (over a 100) but I have never sent these mails. I have tested sending email from my web host and this works perfectly its only when I send mails via zoho mail. I am in dia straights as this is effecting my business but its only affecting my email address jurgen[at]schairer[dot]co[dot]za

Please advise urgently.

Thank you. 

Hi. This is the qmail-send program at mail1302.opentransfer.com. 
I'm afraid I wasn't able to deliver your message to the following addresses. 
This is a permanent error; I've given up. Sorry it didn't work out. 

< mixs[dot]beb[at]hotmail[dot]com >: 
65.54.188.110 does not like recipient. 
Remote host said: 550 Requested action not taken: mailbox unavailable 
Giving up on 65.54.188.110. 

--- Enclosed are the original headers of the message. 


========================= Forwarded message =========================

Subject: Super Quick FastDown Request
From: "Whitney Drake"< jurgen[at]schairer[dot]co[dot]za >
Date: Wed, 4 Nov 2015 05:56:20 +0100
To: < mixs[dot]beb[at]hotmail[dot]com >

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