Mail delivery failed: returning message to sender

So recently this started happening that ONLY the reservation mails are getting spam triggered, as an example:

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:

  [email protected]
    (ultimately generated from [email protected])
    host filter.totaalholding.nl [85.17.242.66]
    SMTP error from remote mail server after end of data:
    550 High probability of spam
  [email protected]
    (ultimately generated from [email protected])
    host filter.totaalholding.nl [85.17.242.66]
    SMTP error from remote mail server after end of data:
    550 High probability of spam
Reporting-MTA: dns; premium15.totaalholding.nl

Action: failed
Final-Recipient: rfc822;[email protected]
Status: 5.0.0
Remote-MTA: dns; filter.totaalholding.nl
Diagnostic-Code: smtp; 550 High probability of spam

Action: failed
Final-Recipient: rfc822;[email protected]
Status: 5.0.0
Remote-MTA: dns; filter.totaalholding.nl
Diagnostic-Code: smtp; 550 High probability of spam
Onderwerp	Nieuwe reserveringsaanvraag
Afzender	‘t zot kiekske
Ontvanger	[email protected]
Datum	Vandaag 04:16
Er is een nieuwe reservering aangevraagd door:
Gereserveerd voor: 03/02/2018 20:00
Reserverende: Rafa?l De Jongh
Met: 3 personen
E-mail: [email protected]
Telefoon: +32485576249

Bekijk nog niet bevestigde reserveringen
Bevestig deze reservering
Weiger deze reservering

As you can see on multiple occasions this mail is getting triggered as 550 High probability of spam even though the source is pretty simple:

Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: from premium15.totaalholding.nl
	by premium15.totaalholding.nl with LMTP id cD9qCZopdVosJgAAboKbHg
	for <[email protected]>; Sat, 03 Feb 2018 04:16:42 +0100
Return-path: <[email protected]>
Envelope-to: [email protected]
Delivery-date: Sat, 03 Feb 2018 04:16:42 +0100
Received: from [2a02:40c0:1000:1000:0:2:37:1] (port=36726 helo=www.tzotkiekske.be)
	by premium15.totaalholding.nl with esmtpsa (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128)
	(Exim 4.89_1)
	(envelope-from <[email protected]>)
	id 1ehoJm-0003KK-23
	for [email protected]; Sat, 03 Feb 2018 04:16:42 +0100
Date: Sat, 3 Feb 2018 03:16:41 +0000
To: [email protected]
From: =?utf-8?Q?=E2=80=98t_zot_kiekske?= <[email protected]>
Reply-To: =?utf-8?Q?Rafa=C3=ABl De Jongh?= <[email protected]>
Subject: Nieuwe reserveringsaanvraag
Message-ID: <[email protected]>
X-Mailer: WPMailSMTP/Mailer/smtp 1.2.4
MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 8bit

<h2>Er is een nieuwe reservering aangevraagd door:</h2>
<p><strong>Gereserveerd voor:</strong> 03/02/2018 20:00<br />
<strong>Reserverende:</strong> Rafa?l De Jongh<br />
<strong>Met:</strong> 3 personen<br />
<strong>E-mail:</strong> <a href="mailto:[email protected]">[email protected]</a><br />
<strong>Telefoon:</strong> <a href="tel+32485346249">+32485346249</a></p>
<p><a href="https://www.tzotkiekske.be/wp-admin/admin.php?page=rtb-bookings&status=pending">Bekijk nog niet bevestigde reserveringen</a><br />
<a href="https://www.tzotkiekske.be/wp-admin/admin.php?page=rtb-bookings&rtb-quicklink=confirm&booking=1072">Bevestig deze reservering</a><br />
<a href="https://www.tzotkiekske.be/wp-admin/admin.php?page=rtb-bookings&rtb-quicklink=close&booking=1072">Weiger deze reservering</a></p>

I do however notice that this is plain html that’s being parsed rather than plain text or a multi-part message in MIME format.

An example of a perfectly working fine e-mail sent via Contact Form 7

Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: from premium15.totaalholding.nl
	by premium15.totaalholding.nl with LMTP id 0JSvE5IKclp5cQAAboKbHg
	for <[email protected]>; Wed, 31 Jan 2018 19:27:30 +0100
Return-path: <[email protected]>
Envelope-to: [email protected]
Delivery-date: Wed, 31 Jan 2018 19:27:30 +0100
Received: from [2a02:40c0:1000:1000:0:2:37:1] (port=60478 helo=www.tzotkiekske.be)
	by premium15.totaalholding.nl with esmtpsa (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128)
	(Exim 4.89_1)
	(envelope-from <[email protected]>)
	id 1egx6Y-0001UH-71
	for [email protected]; Wed, 31 Jan 2018 19:27:30 +0100
Date: Wed, 31 Jan 2018 18:27:30 +0000
To: [email protected]
From: =?UTF-8?Q?Rafa=C3=ABl_De_Jongh_De_Jongh?= <[email protected]>
Reply-To: [email protected]
Subject: =?UTF-8?Q?=E2=80=98t_zot_kiekske_Contact_Formulier:_Dit_is_een_test_beric?=
 =?UTF-8?Q?ht_van_Rafa=C3=ABl?=
Message-ID: <[email protected]>
X-Mailer: WPMailSMTP/Mailer/smtp 1.2.4
MIME-Version: 1.0
Content-Type: multipart/alternative;
	boundary="b1_9addec3a9491059f8744d2b6f9d3c123"
Content-Transfer-Encoding: 8bit

This is a multi-part message in MIME format.

--b1_9addec3a9491059f8744d2b6f9d3c123
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Contact Formulier Van: Rafa??l De Jongh De Jongh
Naam: Rafa??l De Jongh De Jongh
E-mail: [email protected]
Telefoon: +32485346249
Bericht met onderwerp: Dit is een test bericht van Rafa??l
Dit is een test bericht van Rafa??l De Jongh om het contact formulier na te kijken of dat dit iets foutief geeft.

--b1_9addec3a9491059f8744d2b6f9d3c123
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 8bit

<!doctype html>
<html xmlns="https://www.w3.org/1999/xhtml" dir="ltr" lang="en-US">
<head>
<title>a€?t zot kiekske Contact Formulier: Dit is een test bericht van Rafa??l</title>
</head>
<body>
<h3 style="background:#e3c64e;color:#ffffff;padding:4px 8px;font-weight:700;">Contact Formulier Van: Rafa??l De Jongh De Jongh</h3>
<p>Naam: Rafa??l De Jongh De Jongh<br />
E-mail: <a href="mailto:[email protected]?subject=a€?t zot kiekske Contact Formulier: Dit is een test bericht van Rafa??l">[email protected]</a><br />
Telefoon: <a href="tel:+32485346249">+32485576249</a></p>
<h3 style="background:#e3c64e;color:#ffffff;padding:4px 8px;font-weight:700;">Bericht met onderwerp: Dit is een test bericht van Rafa??l</h3>
<p>Dit is een test bericht van Rafa??l De Jongh om het contact formulier na te kijken of dat dit iets foutief geeft.</p>
</body>
</html>

--b1_9addec3a9491059f8744d2b6f9d3c123--

So this might be also related to my HTML template question I posted earlier, as I do note the multipart mail has a proper doctype and html/head/title/body/etc rather than the basic html the reservation outputs. Which I also think is the cause of this problem.

So if you could find some clues on how to fix this that would be great! First thing would probably be to configure it as a multipart mail having both plain text and basic html formatting, perhaps?

Thanks in advance for further information.

PS: I tried various layouts/templates/e-mail addresses, but all get the same error directly after receiving the reservation on my admin’s mailbox.

The page I need help with: [log in to see the link]