I've been in contact with a Eudora user whose mail is failing this test; it
turns out that Mac Eudora allows the X-Mailer header to be rewritten
arbitrarily. Quote:

"To explain what is occurring, I must go into a few details. Windows
Eudora has a Built-In/Hardcoded X-Mailer Header which matches the
string given above (I think). It also provides for the user to code
additional X-Headers in the Eudora.Ini File (where all the Eudora
Settings/Preferences are kept). Here is the docs on the relevant
setting:

ExtraHeaders
Extra headers that are sent with each outgoing message. If multiple
headers are to be sent, separate each with "\r\n".
For example, "ExtraHeaders=X-Header1: foo\r\nX-Header2: bar".

The Macintosh version of Eudora does NOT contain any automatic
X-Mailer Header. If the user wants one to be inserted into their
message, they must use the equivalent "Add X-Headers" feature (which
is to use a Menu Plug-In or <x-eudora-setting:xxxx> [where xxxx is
4401-4499] URL). This is what I do."

He tried out setting X-Mailer to match what we've got, but it still fails:

X-mailer: Eudora Pro 6.0 PPC (MacOS 10.2.8)
Message-id: <p06002002bbb30d17d8af@[192.168.1.11]>
X-spam-status: No, hits=-0.8 required=5.0 tests=BAYES_00,FORGED_MUA_EUDORA,
    FROM_ENDS_IN_NUMS,RCVD_IN_NJABL autolearn=no version=2.70-cvs

Looks like we need to do a similar job on the Eudora rules as we did for Outlook
-- match *just* the Windows Eudora versions we know about.