You need to use the IF or CHOOSE condition.

<xsl:transform version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <out>
      <sendMail>
        <mailFrom>support@company.com</mailFrom>
        <mailTo>johndoe@example.com</mailTo>
        <subject>Email subject</subject>
        <textBody>Email text body</textBody>
        <htmlBody>
          <xsl:choose>
            <xsl:when test="/in/user/email = 'john@somewhere.com'" >
                <h1>First email HTML body</h1>
            </xsl:when>
            <xsl:when test="/in/user/email = 'john@other.com'" >
                <h1>Second email HTML body</h1>
            </xsl:when>
            <xsl:otherwise>
                 <h1>Default email HTML body</h1>
            </xsl:otherwise>
        </xsl:choose>
        </htmlBody>
      </sendMail>
    </out>
  </xsl:template>
</xsl:transform>

I hope this helps! 

Thank you for your help. Its not really what we need.

We do not want to define the email address in the action. We want the action to use the email address on the order in progress and send the email to the order email address. Preferrabley the Shipping Email address

For the mailTo, just specify the user's email like this so it sets it to the email address of the user.

<mailTo><xsl:value-of select="/in/user/email" /></mailTo>