I am getting a few rejected emails undelivered from my Magento 2 install with the following error message or similar, but only about 1% at most of all customers (1000's), this problem does not occur with google/yahoo/outlook/aol or mainstream email customers receiving emails, only certain customers who must have very strict email client/server settings on a personal or business domain address:
550 Messages should have one or no Content-Type headers, not 2.
All of my other email settings are fine (spf etc), but I am struggling with the above. I noticed in the emails being sent out, looking at the email code there are indeed two content-Type headers, one before the head and then one in the head itself, an extract from a recent email is below:
...
Date: Thu, 30 Jan 2020 06:42:17 +0000
MIME-Version: 1.0
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
Subject: Your Order Confirmation
To: Some One <[email protected]>
From: Magento Store <[email protected]>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org=
/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns=3D"http://www.w3.org/1999/xhtml" style=3D"font-size: 62.5%; -we=
bkit-text-size-adjust: 100%; -ms-text-size-adjust: 100%; font-size-adjust: =
100%; background-color: #fff;">
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Dutf-8">
<meta name=3D"viewport" content=3D"initial-scale=3D1.0, width=3Ddevice-widt=
h">
<meta http-equiv=3D"X-UA-Compatible" content=3D"IE=3Dedge">
<style type=3D"text/css">
...
If you are familiar with Magento's Market place, you will find that the emails from the Magento market place also have the two content-type headers after inspecting those emails, so is this problem of concern or is there a solution? Any thoughts!
I use Magento 2.3.4, and Magento's default email templates.