references being converted to mailto links

[James Galvin <galvin+mhonarc-users@xxxxxxxxxx>]

I'm working with a site that recently did an upgrade from version 
2.6.10 to version 2.6.16.  However, an odd thing happened.

Immediately after the cut-over all messages with a references 
header had those elements converted to Mailto: links instead of 
being references to the actually prior messages.

I have looked through the documentation and the archive but nothing 
I found seems applicable in this case.

Have I overlooked something obvious?  Has anyone encountered this 
before?

Any pointers would be appreciated.

For completeness, no special actions were taken for the conversion. 
We just installed and started using v2.6.16.

Thanks!

Jim