Problem: How do I delete large quantities of mail in Eudora?

Solution:

First of all, make sure that the block of mail you want to delete is contiguous. This means that there should not be any mail that you do not want to delete in between the emails you want to delete. Next, follow the instructions below:
1. Open the mailbox where the emails you want to delete are located.
2. Click the last message of the batch you wish to delete (or the first one, whichever one is fine).
3. Using the mouse, scroll to the first message of the batch (or the last one).
4. Press and hold down the SHIFT key and click the first (or last) message. This will select the entire span of messages from first to last, inclusive.
5. Once the messages are selected, you can do one of the following:

(a) Press the Delete key on the keyboard.
(b) Press Apple key + D.
(c) Choose "Delete" from the Message menu.

Note: This actually transfers the "deleted" messages to your TRASH folder.

6. Final step is to empty the Trash folder. You can do this by:

(a) Left click the Trash folder in the mailbox panel, right-click and select Empty Trash
or
(b) Click Special -> Empty Trash on the menu bar.

In the case when Eudora is configured to use IMAP:

Messages -> Purge Messages