Email Bounce Back Errors?

[#1]

we use a collect all email address, which collects emails for 30+ employees.
This week, we have recieved emails from customers saying they're recieving bounce back messages. Here is a copy of one of them:

Quote

Your message did not reach some or all of the intended recipients.       Subject:  RE: Whitlingham       Sent:     11/09/2008 18:31 The following recipient(s) could not be reached:       mspop3connector.username1@bridgesltd.co.uk on 11/09/2008 18:31             The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.             <bridgesltd.co.uk #5.1.1>       mspop3connector.username2@bridgesltd.co.uk on 11/09/2008 18:31             The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.             <bridgesltd.co.uk #5.1.1>       mspop3connector.username3@bridgesltd.co.uk on 11/09/2008 18:31             The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.             <bridgesltd.co.uk #5.1.1>

Can anyone explain this further?
I've a feeling this may be our exchange server

[#2]

probably somebody sent spam from one of your addresses as a spoof address