I was curious about how the Discourse emails are displaying on various email clients. In order to find out, I signed up for Litmus - an email testing service. Litmus gives you a testing email address. I created a user on my forum with that address and then created a post.
With a free account it is only giving me the results for Outlook 2013, the iPhone 6s email client, and the Gmail web client. Here are screenshots from each of those:
It looks as though both Outlook and iPhone are having problems with the ‘visit the topic’ button. On Outlook the button has no padding, on the iPhone the button is split if it goes past the edge of it’s containing block. There are ways to work around both of these issues.
It’s also worth noting that in both Outlook and Gmail there is no maximum width set on the email’s content. On iPhone with the iOS mail app, there is no padding between the content and the edge of the screen.