Problem: I need to compact my mailboxes. How do I do that?

Solution:

Mailboxes in Eudora for Macintosh are composed of two general parts: the mail messages and the table of contents. The table of contents is an index of the mailbox file which keeps track of where in the mailbox file each message is stored and the status (read/unread/forwarded/etc) and priority of each message. When you delete a message, you are actually removing the listing for that message from the table of contents. The message itself is still stored in the mailbox file and takes up space. To actually remove the message, Compact the mailbox. Compacting mailboxes reconciles the mailbox with its table of contents.

You can compact all the mailboxes by holding down on the option key and clicking on the numbers in the lower left-hand corner of an open mailbox window.