Try this:

        <subsystem xmlns="urn:jboss:domain:logging:3.0">

            <async-handler name="EMAIL">

                <level name="ERROR"/>

                <queue-length value="100"/>

                <overflow-action value="discard"/>

                <subhandlers>

                    <handler name="emailHandler"/>

                </subhandlers>

            </async-handler>

            <custom-handler name="emailHandler" class="org.apache.log4j.net.SMTPAppender" module="org.apache.log4j">

                <level name="ERROR"/>

                <formatter>

                    <pattern-formatter pattern="%d{HH:mm:ss,SSS} %-5p [%c] (%t) %s%E%n"/>

                </formatter>

                <properties>

                    <property name="BufferSize" value="512"/>

                    <property name="To" value="app-exception@example.com"/>

                    <property name="From" value="sysadmin@example.com"/>

                    <property name="Subject" value="[${jboss.node.name}] Application Error"/>

                    <property name="SMTPHost" value="localhost"/>

                </properties>

            </custom-handler>

            <root-logger>

                <level name="WARN"/>

                <handlers>

                    <handler name="EMAIL"/>

                </handlers>

            </root-logger>

        </subsystem>

I'd suggest using CLI instead of editing the XML. Something like this should achieve the same results and not require shutting down WildFly and restarting it.

/subsystem=logging/custom-handler=emailHandler:add(class="org.apache.log4j.net.SMTPAppender", module="org.apache.log4j", level=ERROR, named-formatter=PATTERN, properties={BufferSize="512", To="app-exception@example.com", From="sysadmin@example.com", Subject="[${jboss.node.name}] Application Error", SMTPHost="localhost"})
/subsystem=logging/async-handler=async:add(queue-length=100, overflow-action=discard, handlers=[emailHandler])
/subsystem=logging/root-logger=ROOT:add-handler(name=async)

--

James R. Perkins