Steps to replicate the issue (include links if applicable):

• Run a bot task
• Wait for task to complete
• Receive mail with result

What happens?:
In some mail clients (e.g. roundcube) show the raw HTML and not the parsed HTML

What should have happened instead?:
The parsed HTML should be shown

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

I see MIME Version and Content-Type Headers twice:
MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1