[imp] Warning banner in some messages

[Michael Dahlberg]

I recently upgraded our horde installation to horde 3.1.3, imp 4.1.3,  
turba 2.1.2, and ingo 1.1.1 (we're using the latest apache 2 series  
and php 5.1.6).  Since the upgrade some messages have the following  
banner added to the top of the message in an orange box:

Warning: This message may not be from whom it claims to be. Beware of  
following any links in it or of providing the sender with any personal  
information.

I can't seem to find any documentation on this feature, why it is  
added, what horde module is adding it and how it can be controlled.   
Could you point me to any info about this feature?

Thanks,
Mike