Hopefully, this isn't a hijack, but...
I'm seeing very similar problems after upgrading to Tahoe last night. The message bodies of my (non-HTML/CSS) mail are now overwhelmingly in some small black condensed font. I can make it bigger with ⌘+, but I'd really like to get rid of the freakish font.
Mail Settings say my body text is Helvetica 14, but apparently it really isn't anymore. This behavior has also changed the appearance of old, saved mail from before the Tahoe upgrade (see image 2).
Searching for "font" in System Settings doesn't show anything useful, except for a table in which it advises that Mail controls its own.
(Image show WAY larger than size on screen)
Mail from today:
Mail saved from days ago:
