code: public static String reverseDomain(String user) {
  		StringBuilder sb = null;
  		String[] split = Split.split('.',user);
  		int at;
  		for(int i=split.length-1;i>=0;--i) {
  			if(sb == null) {
  				sb = new StringBuilder();
  			} else {
  				sb.append('.');
  			}
  
  			if((at = split[i].indexOf('@'))>0) {
  				sb.append(split[i].subSequence(at+1, split[i].length()));
  			} else {
  				sb.append(split[i]);
  			}
  		}
  		
  		return sb==null?"":sb.toString();
  	}
docs: Take a Fully Qualified User, and get a Namespace from it.
  @param user
  @return