Getting Bombarded By Emails

Question

My email is configured as follows:

I have a gmail account under my name, which is my primary email account. My gmail is then set to check the email account "[e-mail]", and retrieve any emails from this account, and copy them to my inbox. This is a website I run, and the email address is configured as an account on a POP mail server, using Mail Enable as the server.

Yesterday, I looked in the Trash folder of my gmail account, and found the following email:

The message "Emailing: Image (8239).pdf" from [e-mail] contained a virus or a suspicious attachment. It was therefore not fetched from your account [e-mail] and has been left on the server.

Message-ID: <[e-mail]>

If you wish to write to this person, just hit reply and send a message.


Thanks,

The Gmail Team

The header info is as follows:

from:   Gmail Team <[e-mail]>
reply-to:   [e-mail]
to: Gavin Coates <[e-mail]>
date:   Wed, Jul 6, 2016 at 8:50 AM
subject:    Emailing: Image (8239).pdf
mailed-by:  google.com

I am receiving one of these messages every second!

They appear to come from a handful of addresses, in each case they are domains I own that are hosted on the same server. The account name varies, and sometimes is an email address that exists/have used in the past, but sometimes is an email I've never used.

I've now changed the password for the webmaster account, and have not updated this in gmail (it now shows authentication failed in the settings), yet I'm still receiving the emails.

I'm a little concerned by these emails. Why am I receiving so many (literally 1 per second)? Are these emails being sent to me, or are they being sent from my server?

Any help in identifying the cause of these would be greatly appreciated!

EDIT: I've disabled the email account [e-mail], and the mail server under that domain, but they still appear. Here is a sample of one of the messages received on that account prior to disabling:

Received: from dynamic.vdc.vn ([113.190.165.17]) by home with MailEnable ESMTP; Mon, 4 Jul 2016 18:51:56 +0100
From: <[e-mail]>
To: <[e-mail]>
Subject: Emailing: Image (8239).pdf
Date: Tue, 05 Jul 2016 00:51:45 +0700
Message-ID: <[e-mail]>
MIME-Version: 1.0
Content-Type: multipart/mixed;
    boundary="----=_NextPart_000_001F_4E2DB17B.F72AFC55"
X-Mailer: Microsoft Office Outlook 12.0
Content-Language: en-gb
Return-Path: <[e-mail]>