Is someone sending span via my domain smtp, which uses zohomail

Is someone sending span via my domain smtp, which uses zohomail

Hello everyone, I do not know what's happening. I have a zohomail account linked to my wgbn.com.br domain in which I use it for personal use.

I've been receiving mesnasgens from "Mail Delivery System" reporting an error in the delivery of some emails. Emails the ones I did not send. The content of these emails are phishing material and links to sites with malicious content.

How can this be happening? How to protect my SMTP account from sending unauthorized out?

The email:

This is the mail system at host server2.hostingforyou.de. 

I'm sorry to have to inform you that your message could not 
be delivered to one or more recipients. It's attached below. 

For further assistance, please send mail to postmaster. 

If you do so, please include this problem report. You can 
delete your own text from the attached returned message. 

The mail system 

<johnspendelow@web.de> (expanded from <wardvawody51@johnspendelow.de>): host 
mx-ha02.web.de[212.227.17.8] said: 550 Requested action not taken: mailbox 
unavailable (in reply to RCPT TO command) 
Reporting-MTA: dns; server2.hostingforyou.de 
X-Postfix-Queue-ID: 0A8C93017D 
X-Postfix-Sender: rfc822; walter@wgbn.com.br 
Arrival-Date: Wed, 17 May 2017 19:11:29 +0200 (CEST) 

Final-Recipient: rfc822; johnspendelow@web.de 
Original-Recipient: rfc822;wardvawody51@johnspendelow.de 
Action: failed 
Status: 5.0.0 
Remote-MTA: dns; mx-ha02.web.de 
Diagnostic-Code: smtp; 550 Requested action not taken: mailbox unavailable 

Received: by server2.hostingforyou.de (Postfix) 
    id 0A8C93017D; Wed, 17 May 2017 19:11:29 +0200 (CEST)
Delivered-To: web43_info@server2.hostingforyou.de
Received: from [196.216.220.10] (unknown [196.216.220.10]) 
    by server2.hostingforyou.de (Postfix) with ESMTP id 54FA530093 
    for <wardvawody51@johnspendelow.de>; Wed, 17 May 2017 19:11:28 +0200 (CEST)
Message-ID: <BDC844433231CF3540CCCBBAB9474A95@GJB6XCI7A>
From: <walter@wgbn.com.br>
To: <wardvawody51@johnspendelow.de>
Subject: 65750
Date: 17 May 2017 16:03:18 -0100
MIME-Version: 1.0
Content-Type: multipart/alternative; 
    boundary="----=_NextPart_000_0025_01D2CF30.07C51283"
X-Priority: 3
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook Express 6.00.2900.5931
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5994