What would cause this iMap error on my Exchange box? (users on Macs using Thunderbird)

I have a Server 2003 R2 Standard box running Exchange 2003 (V. 6.5.7638.1).  A couple of my Mac users have recently started getting the attached error when retreiving their email with Thunderbird.  One user gets it when he opens Thunderbird while another user gets the error each time he opens a message.  When it happens, the attached iMap error is generated in the server event logs.  

I understand that the error is because the messages failed to correctly convert to text but what could be suddenly causing it so consistantly?

Thanks! TBird-Error.tiff
application-log.docx