Multipart emails via mailgun not working

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I am trying to set up sending of multipart emails for improved email deliverability.

I set this up following instructions here, substituting wp_mail for mail: http://webcheatsheet.com/PHP/send_email_text_html_attachment.php

This works when the WP Mail SMTP plugin is disabled but results in an error when it is enabled.

Does WP Mail SMTP support the sending of multipart emails via Mailgun? If so how can I get this to work?

Thanks,

John