You may be activating a font suitcase that contains Helvetica Fractions. There is a known issue where if a Helvetica Fractions is ever available in the system (probably as part of a complete Helvetica suitcase active within Font Reserve) Mail, Safari, Addressbook, and any other application that displays html will opt to use the fractions rather than the defaultHelvetica. Close this font, and relaunch Mail and things should appear normally again. If this is not the case, you'll need to trash your font cache files. OS X uses cache files to track fonts. If these cache files get corrupted, fonts will often display incorrectly. To fix this type of problem, you need to trash all cache files related to font handling.There are a few free programs out that will clear font cache files, including Font Cache Expunger, Cache Out X, and Onyx. Read the rest of the hint for Extensis' instructions on manually deleting cache files in the Terminal...
[robg adds: This hint is an expansion on an earlier hint which didn't cover all of the various cache files. I modified the formatting of Extensis' instructions in the following explanation, but did not change any of the actual content...]
How to remove Font Cache files directly in OS X (Panther):
First, launch the Terminal (found in /Applications/Utilities). These instructions are CASE SENSTIVE. This means that UPPERCASE and lowercase are important to remember. In the Darwin environment of Mac OS X, two files can be named identically but be different files if the case of the filename is different, (i.e., 'FILENAME' and 'FileName' would be two different files.)
After each command be sure to press Return. These instructions assume that you are an Admin user in OS X. If you are not, or this is a shared use machine, please contact your administrator for assistance. This is usually easiest if you copy and paste each of the commands to type. If you receive a "No Match" or "File not Found" error, continue on, not everyone will have all of these files in all locations.
You'll be presented with a prompt similar to:
Welcome to Darwin! [hostname:~] username$At the $ prompt type the following (followed by Return): cd ~/Library/Caches. The prompt should change and say:
[hostname:~/Library/Caches] username$Next, at the $prompt type in: sudo rm *.fcache*. You should be prompted for a password. Go ahead and enter your password followed by Return. You'll be presented with the prompt if it was successful. Next, at the $prompt, type in: sudo rm *.FODB* and sudo rm *fontcache*. Now at the $prompt type in cd /Library/Caches; the prompt should change and say:
[hostname:/Library/Caches] username$Again, type in: sudo rm *.fcache*, also sudo rm *.FODB* and sudo rm *fontcache*. Now, our last stop for cache files: cd /System/Library/Caches. The prompt should change and say:
[hostname:/System/Library/Caches] username$Now type in sudo rm *.fcache* and of course sudo rm *.FODB* and the last file to remove sudo rm fontTablesAnnex*. Now you can select 'Quit' from the Terminal menu and restart your computer.

