replace-email.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<!-- This template will return the Message, changing the email address to the one provided. -->   
    <xsl:output method="xml" omit-xml-declaration="yes"/>
    
    <!-- Here we set up a parameter that will be passed in. -->
    <xsl:param name="emailReplacement"/>
    
    <!-- This block copies the whole thing (the input) into the output. -->
    <xsl:template match="node() | @*">
        <xsl:copy>
            <xsl:apply-templates select="node() | @*"/>
        </xsl:copy>
    </xsl:template>

    <!-- This block replaces the `email_addr` atribute of the Email element with the given email. -->
    <xsl:template match="/Message/Email/@email_addr">
        <!-- The name attribute is required, so we just specify the same name as before. `name()` uses the same name the attribute had previously—we don't want to change the attribute name, just its value. -->
        <xsl:attribute name="{name()}"> 
            <xsl:value-of select="$emailReplacement"/>
        </xsl:attribute>
    </xsl:template>
    
</xsl:stylesheet>