Eudora applies SpamSieve to all incoming messages. The Junk Mail area of Eudora’s preferences lets you customize how Eudora interacts with SpamSieve. Note that the Junk Threshold slider will have no effect because SpamSieve always considers messages with scores of 50 or above to be spam. Thus, you should set Eudora’s junk threshold to 50 and use the slider in the Advanced tab of SpamSieve’s preferences if you need to adjust its sensitivity.
Other settings in the Junk Mail pane do affect SpamSieve. For instance, if you check Mail isn’t junk if the sender is in an address book, then Eudora will not pass those messages along to SpamSieve; it will assume that they are good. Note that Eudora always considers your address to be in its address book, even though it may not be explicitly listed there. Thus, if you receive spam that is forged so that it appears to be sent from your own address, you must uncheck Mail isn’t junk if the sender is in an address book in order for SpamSieve to catch it.
To still have SpamSieve whitelist your address book, you can export the Eudora address book to vCard and then import it into the Mac OS X Address Book, or use SpamSieve’s Import Addresses… command to import the Eudora Nicknames file.
The Junk Extras area of Eudora’s preferences lets you control some additional settings, such as whether junk messages are removed from the mail server.