Yes this is very common now, the hackers use phising and straight
  guessing to get accounts and then send spam.
  Some settings you may find useful are listed here, we've tried to add a
  lot of tools to make it possible to protect yourself and to get a warning when
  it happens.
  Be aware some measures will occasionally cause your own users confusion
  so you have to balance how much protection you want with how much your users
  can cope with :-)  
  ChrisP.
  
    just got a call from a customer. he's getting a huge number of
    non-delivery notices for emails he did not send. 
     
    none of the 'to' addresses are in his address book so it's not a trojan
    or virus on his workstation. 
     
    i looked at a few of the returned messages and they all look like this:
    
     
    X-Default-Received-SPF: pass (skip=loggedin (res=PASS))
    x-ip-name=77.222.42.120; THIS IP IS DIFFERENT ON EACH MSG 
    Date: Thu, 28 Jun 2012 21:30:40 +0300 
    From: Paul DeLay <HIDDEN@r@onebrainmarketing.com> THE NAME IS
    DIFFERENT ON EACH MSG 
    Organization: mbpdsy 
    X-Priority: 3 (Normal) 
    Message-ID: <744914006HIDDEN@28213040@onebrainmarketing.com>
    
    To:HIDDEN@baker884.fsnet.co.uk 
    Subject: Look at Pic No. 776 
    MIME-Version: 1.0 
    Content-Type: text/plain; charset=us-asciislplavsic 
    Content-Transfer-Encoding: 8bit 
    X-Authenticated-User:HIDDEN@r@onebrainmarketing.com 
     
    then there's some nasty text. 
     
    i had him change his password immediately. 
     
    looking at the outbound queue, there are still a few message from him
    awaiting delivery. they all have different 'from' ip addresses. i've deleted
    them. 
     
    since we're very strict about requiring authentication for smtp, the
    only thing i can think of is that his password was guessed. 
     
    anyone have any ideas as to how this can be prevented - other than
    strong passwords? 
     
    david camm 
    advanced web systems 
    keller, tx