Emails being sent to Mandrill as text instead of html

[tprocter1984]

Magento version: 2.2.9 Module version: 102.3.1

There is an issue where the "New contact form submission " email is being sent as TEXT content instead of HTML.

I have spoken to Mandrill and asked if they could force the emails to be HTML from their side, and they have responded with:

There is not a way to force all messages sent through Mandrill to be HTML. This is ultimately up to the proper parameters being sent by the API/SMTP integration. Wish I had better news for you on this. I'm glad to provide this feedback to our engineers though.

The header from the recieved email looks like:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mandrillapp.com;
  i=@mandrillapp.com; q=dns/txt; s=mandrill; t=1586950653; h=From :
  Subject : To : Message-Id : Date : MIME-Version : Content-Type : From :
  Subject : Date : X-Mandrill-User : List-Unsubscribe;
  bh=EZbSEgyG747SNxDMMDsWuyVynBIieFLqNhbI7Tqssno=;
  b=nFJGDwruMjdU2zB+9uc7O3EDG/+xU4Fkp1OKjPaBNDs7ZSZEDuoX3RPUaEUKeh87uXMicg TBwgijcxqgetafuYEJFtlAgiSGLssi2Zh31iMgjUQAByfGBJ2n4idTs8m+UBfwdHRKdK4tht E/93bqoPGbN7a6t+EDZubNv5uNx0g=
From: email@email.com
Subject: New contact form submission
Return-Path: <bounce-md_30995146.5e96f1fd.v1-4ca3d596d51044fabcb08ce6c9dc2b58@mandrillapp.com>
Received: from [185.53.174.244] by mandrillapp.com id 4ca3d596d51044fabcb08ce6c9dc2b58; Wed, 15 Apr 2020 11:37:33 +0000
To: email@email.com
X-Report-Abuse: Please forward a copy of this message, including all headers, to abuse@mandrill.com
X-Report-Abuse: You can also report abuse here: http://mandrillapp.com/contact/abuse?id=30995146.4ca3d596d51044fabcb08ce6c9dc2b58
X-Mandrill-User: md_30995146
Message-Id: <30995146.20200415113733.5e96f1fdb24c13.30496083@mail186-29.suw21.mandrillapp.com>
Date: Wed, 15 Apr 2020 11:37:33 +0000
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="_av-LFbv8noilKNonqLOqqR2ng"

--_av-LFbv8noilKNonqLOqqR2ng
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit

Any help greatly appreciated!

Tom