How to replace/overwrite default header of EmailMultiAlternatives

Question

Environment: Ubuntu 18.10, Python 2.7.15, Django 1.11.16

I'm trying to send an email containing an inline image. I have the following code:

msg = EmailMultiAlternatives(some_subject, some_body, 'from@some-domain.com', ['to@some@domain'])

img_data = open('path/to/image.png', 'rb').read()
img = MIMEImage(img_data)
msg.attach(img)

msg.send()

(I've only included the code that I think is relevant but I can add more on demand.)

The above properly works and the image is properly displayed on most of the email clients (about 7 of them, both mobile, desktop or webmail ones) that I tested on, with two exceptions: Mozilla Thunderbird 60 and some macOS native email client.

On Thunderbird the image is not displayed inline but at the very end of the message. On the macOS client, the image is displayed inline but additionally it is also displayed at the very end of the message.

I composed and sent a test message from another email client, containing an inline image which was properly displayed on both Thunderbird and macOS. I compared the headers of this message with the headers of the message generated by my code.

I noticed that the faulty message has the 'Content-Type' set to 'multipart/mixed' while the properly displayed message had the same header set to 'multipart/related'.

I saved the faulty message in an eml file and manually changed the value of that header and then loaded the message in Thunderbird. The message was properly displayed and the image was in the right place.

If I could set that header to the proper value, the problem would be solved.

So, my question is: is there any possibility to tell EmailMultiAlternatives to set 'Content-Type' : 'multipart/related' instead of the default value of 'multipart/mixed'?

I tried to add the header like this but it is not working:

msg = EmailMultiAlternatives(some_subject, some_body, 'from@some-domain.com', ['to@some@domain'], headers={'Content-Type' : 'multipart/related'})

I got the following error ( I use Amazon SES):

400 Bad Request
<ErrorResponse xmlns="http://ses.amazonaws.com/doc/2010-12-01/">
  <Error>
    <Type>Sender</Type>
    <Code>InvalidParameterValue</Code>
    <Message>Duplicate header 'Content-Type'.</Message>
  </Error>
  <RequestId>xxxxxxxxxx</RequestId>
</ErrorResponse>

If I can't modify that header, do you suggest any alternatives?

Answer 1

If you look at the source code, you'll see that EmailMultiAlternatives is a subclass of EmailMessage, which itself has a class attribute:

mixed_subtype = 'mixed'

So if you create your own subclass to override this, you should get what you need:

class EmailMultiAlternativesRelated(EmailMultiAlternatives):
    mixed_subtype = 'related'

That's it, now you just use this new class, and it will use "multipart/related".

(the _create_attachments() method passes this subtype to python's SafeMIMEMultipart which creates the actual headers for each attachment.)