I attached the file issueresolved.vm, where I made the changes to view the custom field (Freetext with wiki renderer) 10150:

  #if($issue.getCustomFieldValue("customfield_10150"))
    <tr bgcolor="#ffffff"><td>
        <b>Analyse:</b> <br/>
	<pre>
        $issue.getCustomFieldValue("customfield_10150")
	</pre>
    </td></tr>
  #end

The custom field contains e.g. the following entry in wiki notation:

Line 1: *bold text*

Line 3: {{monospaced text}}

The way it is implemented right now renders the following output:

However, I want the output to be rendered ideally with the renderer configured for this field. If that does not work, I would also hardcode the wiki renderer. (As for my understanding, the wiki-text must somehow be translated to HTML, so that it is correctly inserted into the HTML eMail.

Reference: http://confluence.atlassian.com/display/JIRA/Adding+Custom+Fields+to+Email