Mail Validator NoClassDefFoundError

Transformation with Mail Validator step runs fine in Spoon but when called from Pentaho User Console -> Action Sequence -> Job -> Transformation produces this error:

2015-06-16 14:13:32,443 ERROR [org.pentaho.di] Mail Validator - Unexpected error
2015-06-16 14:13:32,443 ERROR [org.pentaho.di] Mail Validator - java.lang.NoClassDefFoundError: org/apache/commons/validator/GenericValidator
at org.pentaho.di.trans.steps.mailvalidator.MailValidation.isRegExValid(MailValidation.java:43)
at org.pentaho.di.trans.steps.mailvalidator.MailValidation.isAddressValid(MailValidation.java:142)
at org.pentaho.di.trans.steps.mailvalidator.MailValidator.processRow(MailValidator.java:157)
at org.pentaho.di.trans.step.RunThread.run(RunThread.java:40)
at java.lang.Thread.run(Thread.java:722)


I assume I need to reference commons-validator-1.3.1.jar somewhere but where? I've tried CLASSPATH, catalina.properties but to no avail.

Environment:
Pentaho Data Integration 4.2.0-stable
Windows Server 2008 R2 Standard
Java(TM) SE Runtime Environment (build 1.7.0_03-b05)