MX4J is released under an Apache-style license. In practice this means that you can do almost anything you want with the code, including its use in commercial software.
The actual text of the license is included below:

 The MX4J License, Version 1.0
 Copyright (c) 2001-2004 by the MX4J contributors. All rights reserved.
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1. Redistributions of source code must retain the above copyright
 notice, this list of conditions and the following disclaimer.
 2. Redistributions in binary form must reproduce the above copyright
 notice, this list of conditions and the following disclaimer in
 the documentation and/or other materials provided with the
 distribution.
 3. The end-user documentation included with the redistribution,
 if any, must include the following acknowledgment:
 "This product includes software developed by the
 MX4J project (http://mx4j.sourceforge.net)."
 Alternately, this acknowledgment may appear in the software itself,
 if and wherever such third-party acknowledgments normally appear.
 4. The name "MX4J" must not be used to endorse or promote
 products derived from this software without prior written
 permission.
 For written permission, please contact
 biorn_steedom [at] users [dot] sourceforge [dot] net
 5. Products derived from this software may not be called "MX4J",
 nor may "MX4J" appear in their name, without prior written
 permission of Simone Bordet.
 THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 DISCLAIMED. IN NO EVENT SHALL THE MX4J CONTRIBUTORS
 BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 SUCH DAMAGE.
 ====================================================================
 This software consists of voluntary contributions made by many
 individuals on behalf of the MX4J project. For more information on
 MX4J, please see
 the MX4J website.