input keeps growing, below is one of the file contents: This message was created automatically by mail

The number of files in /var/spool/exim/input keeps growing, below is one of the file contents:

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. The following address(es) failed:

some.email@gmail.com
SMTP error from remote mail server after end of data:
host gmail-smtp-in.l.google.com [2607:f8b0:4003:c11::1b]:
550-5.7.1 [2600:3c00::f03c:91ff:fe73:ba3e 12] Our system has detected that
550-5.7.1 this message is likely unsolicited mail. To reduce the amount of spam
550-5.7.1 sent to Gmail, this message has been blocked. Please visit
550 5.7.1 support.google.com/mail/answer/188131 for more information. q13si1478836otq.289 - gsmtp

------ This is a copy of the message, including all the headers. ------

Return-path: <lynette_flowers@mysite.technology>
Received: from admin by MySite Technology-USA1 with local (Exim 4.72)
(envelope-from <lynette_flowers@mysite.technology>)
id 1bS4w4-0002yc-EP
for some.email@gmail.com; Tue, 26 Jul 2016 11:10:24 -0500
To: some.email@gmail.com
Subject: Hardcore Spam Email Example Subject X-PHP-Originating-Script: 501:files31.php(1941) : eval()'d code
Date: Tue, 26 Jul 2016 16:10:24 +0000
From: Lynette Flowers <lynette_flowers@mysite.technology>
Message-ID: <b57d5ffdc2572a8a845cf8b0dd81d87b@mysite.technology> X Priority: 3 MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="b1_b57d5ffdc2572a8a845cf8b0dd81d87b"
Content-Transfer-Encoding: 8bit

--b1_b57d5ffdc2572a8a845cf8b0dd81d87b
Content-Type: text/plain; charset=us-ascii

***** NSFW Content Removed *****
***** NSFW Content Removed *****
***** NSFW Content Removed *****

I think someone is using my domain name as their email address (such as in the Return-path) to send spam emails. So the mails are returned to my server from Hotmail, Gmail, etc.

If I don't do anything, my disk space is totally eaten up after a few days and I also see CPU is wasted in exim processing.

How to stop and fix this?