Gmail allows to adds call for actions in messages. They are shown at the right of the message line in the inbox.

GitHub recently added View Issue, View Commit and View Pull Resquest actions for example.

It uses a standard format, describable as microdata. Yet, I'm unaware of implementation in other mail clients.

Interest: Often, a mail is mainly a notification call see this URL. In such cases, we can provide a clear button linking to this URL.

Cost: It becomes more complex to send a mail, as mails become multipart, text/plain for the actual mail, an HTML copy to display the action

Impact: average to high, considering Gmail and Google for Applications market share.

Suggested roadmap:

• Adds code in Remarkup to offer a plain text output capability
• Use Remarkup for outgoiing mail messages
• Create new classes to represent a multipart text / HTML with action message

Example of action code:

<div itemscope itemtype="http://schema.org/EmailMessage">
  <div itemprop="action" itemscope itemtype="http://schema.org/ViewAction">
    <link itemprop="url" href="https://secure.phabricator.com/T6237" />
    <meta itemprop="name" content="View task" />
  </div>
  <meta itemprop="description" content="T6237 - Use ApplicationTransactions properly in Diff creation" />
</div>