The manual is well worth reading. It's illustrated, comprehensive, and over a hundred pages long.
The HyperCard Q&A stack discusses some of the more esoteric features of Eudora, and explains how to modify them with ResEdit.
The Attributions plug-ins change the strings that Eudora inserts before a quote. The default is nothing for a reply to the sender, and "At <time> on <date>, <sender> wrote:" for a reply to all recipients. These four plug-ins provide four variations on that. The Finder's Get Info box of each tells what it does, so you can keep them straight.
The DN headers plug-in makes Eudora use domain names instead of numerical IP addresses in the routing headers.
The Extra Headers plug-in adds a panel to Eudora's Settings dialog, which allows you to enter additional headers to be added to all your outgoing mail.
The Kill X-Sender plug-in suppresses the "X-Sender: email@example.com" header that Eudora adds if your POP account and your return address aren't the same. (This is not intended for, nor is it any real use in, spoofing mail. Still, be aware that spoofing is easy to do. If you want any real assurance that mail came from who it says it did, use PGP.)
The Peter's quote plug-in changes the quote string from the default ">" to "> " which I find much easier to read.
My new Sig picker plug-in lets you specify a signature folder name other than "Signature Folder," allowing you to have more than one Settings file in the same Eudora Folder, but have them use individual signatures.
Enjoy these, and let me know if you have questions or if you would like other similar plug-ins, or just tell me how you like them.
Wagner Truppel's Wagner's prefs has settings for many useful parameters, including tabs, wrapping, splitting large messages, mailbox compacting, and quote characters.
Brad Pillow's Nickname Settings lets you change the width of the Nicknames window, so you can see longer names.
Andy Fragen's RSVP prefs allows you to change the From: headers from the default "firstname.lastname@example.org (Real Name)" to "Real Name <email@example.com>".
Robert Rosenberg's Blah/Progress Settings can change the default message display from full to abbreviated headers, disable the progress window when sending and receiving mail, and adjust the number of spaces per tab.
Dave Del Torto's Eudora Goodies directory has plug-ins and other useful Eudora tools. (People occasionally write to tell me that this link is dead. Keep trying. Last I checked, it wasn't dead, just Nyetcom.)
Check out the Eudora Tricks & Tips pages for more Eudora information and utilities, for Macintosh and Windows.