It requires the following features: send-mail.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<!-- Clear the Sendria log -->
<p:http-request href="http://localhost:1080/api/messages/" method="delete">
<p:with-input>
<p:empty/>
</p:with-input>
</p:http-request>
<!-- Make sure that succeeded -->
<p:if test=".?code != 'OK'">
<p:error code="Q{{http://example.com/}}irrelevant">
<p:with-input>
<p:empty/>
</p:with-input>
</p:error>
</p:if>
<p:send-mail parameters="map{'host':'localhost', 'port':1025}" auth="map{'username':'username','password':'password'}">
<p:with-input>
<p:inline>
<emx:Message xmlns:emx="URN:ietf:params:email-xml:" xmlns:rfc822="URN:ietf:params:rfc822:">
<rfc822:from>
<emx:Address>
<emx:adrs>mailto:user@example.com</emx:adrs>
<emx:name>Example User</emx:name>
</emx:Address>
</rfc822:from>
<rfc822:to>
<emx:Address>
<emx:adrs>mailto:anotheruser@example.com</emx:adrs>
<emx:name>Another User</emx:name>
</emx:Address>
</rfc822:to>
<rfc822:subject>Multi-part Email</rfc822:subject>
<emx:content type="text/plain">
This is my text message.
</emx:content>
</emx:Message>
</p:inline>
<p:inline>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Hello, World</title>
</head>
<body>
<h1>Hello, World!</h1>
<p>This is my attachment.</p>
</body>
</html>
</p:inline>
</p:with-input>
</p:send-mail>
<!-- Check the Sendria log -->
<p:http-request href="http://localhost:1080/api/messages/" method="get"/>
<p:http-request href="http://localhost:1080/api/messages/" method="get"/>
<p:http-request href="http://localhost:1080/api/messages/" method="get"/>
<p:cast-content-type content-type="application/xml"/>
</p:declare-step>
Schematron validation
<s:schema xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:ns prefix="fn" uri="http://www.w3.org/2005/xpath-functions"/>
<s:pattern name="Root must be map">
<s:rule context="/fn:map">
<s:assert test="fn:string[@key='code'] = 'OK'">Code must be 'OK'</s:assert>
<s:assert test="fn:array[@key='data']/fn:map/fn:string[@key='subject'] = 'Multi-part Email'">The subject must be correct</s:assert>
<s:assert test="fn:array[@key='data']/fn:map/fn:string[@key='type'] = 'multipart/mixed'">The content type must be multipart/mixed</s:assert>
</s:rule>
</s:pattern>
</s:schema>