Launch Font Book from the Applications menu, select all the Fonts and from the menu bar, select Validate Font.
This will run a check on all the fonts and advise you if there are any that could be causing a problem.
You can then delete the offending font.
As both machines got the same error, from the Mail, it is probably just a problem in one of the emails you received.
I have an intel Macbook and a PPC iMac both running latest version Tiger. First the iMac then the laptop developed multiple (one on top of the other messages warning that there was an error detected with the $C font. It suggested I should quit all apps and try to remove the font. I think this happened in Mail on both machines. What should I do? Is it a piece of malware? Strange that it should happen on both machines.
Thanks for any help out there