Why am I getting this null pointer exception?

Question

I keep getting a null pointer exception and I am not sure why. I looked at what causes this error but I still can't figure it out. I think after this error it may not be right but I can fix that after but first I have to fix this. This is the error I am getting:

 ----jGRASP exec: java PostOfficeTester
Exception in thread "main" java.lang.NullPointerException
at PostOffice.retrieveMsg(PostOffice.java:33)
at PostOffice.placeLetter(PostOffice.java:17)
at PostOfficeTester.main(PostOfficeTester.java:11)

----jGRASP wedge: exit code for process is 1.
----jGRASP: operation complete.

Here is code:

public class PostOfficeTester{
public static void main(String[] args)
{
  PostOffice branch = new PostOffice(5);
  branch.retrieveMsg(5);
  branch.placeLetter(new Letter("Arnold", "I'll be back!"),5);
  branch.placeLetter(new Letter("Spock", "Live long and prosper."),4);
  branch.placeLetter(new Letter("Kirk", "KHAAAAAN!"),4);
  branch.retrieveMsg(4);
  branch.retrieveMsg(2);
  branch.retrieveMsg(-1);
  branch.placeLetter(new Letter("Harry", "Make my day!"),0);
  branch.placeLetter(new Letter("Ellie", "Thanks for the adventure! Now go have another one!"), 1);
  branch.placeLetter(new Letter("Buzz", "To infinity... and beyond!"), 3);
  branch.findSender("Kirk");
  branch.findSender("Ellie");
} 
}

 public class PostOffice {

 private Letter pobox[];

 public PostOffice(int size){
    pobox = new Letter[size];
 }

 public boolean placeLetter(Letter mail, int boxNum){
    if(boxNum>=pobox.length || (!(retrieveMsg(boxNum).equals("Empty!")))){   
      return false;
    }
    else{
       return true;
    }
 }

 public String retrieveMsg(int boxNum){
    if(boxNum>=pobox.length){
       return "Box does not exist!";
    }

    else if(pobox[boxNum].equals("")){    
       return "Empty!";
    } 
    else{
       return pobox[boxNum].getMsg();
    }     
 }

 public Letter findSender(String name){
    for(int i = 0; i<pobox.length; i++){
       if(name.equals(pobox[i].getSender())){
          return pobox[i];
       }
     }
     return null;        
   }

  } 

public class Letter {

   private String sender;
   private String message;

   public Letter(String name, String text){
      sender = name;
      message = text;
   }

   public String getSender(){
      return sender;
   }

   public String getMsg(){
      return message; 
   }
  }