The open source version of eudora is not supported
I don't think that excerpt above is referring to the open source version (which has never been supported, AFAIK).
I'm more concerned with Outlook Express, though.
Yes, I was confused by that same section as well. About Eudora and Outlook Express, and then also by this:
Here is a list of email clients that are not an were not ever supported as they are known to lack the necessary AppleEvent support for external control (this is not an exhaustive list)...
Eudora OSE (Open Source Edition)
Thunderbird has long worked perfectly with FileMaker's Send Mail step, at least on Windows. That is what we're talking about here, right? Or is this non-support referring to Mac only, since it refers to "AppleEvent support"?