In Yahoo mail, how do I 'un-spam' an e-mail I mistakenly marked as 'spam'?

You can recover the email from your trash. You also need to make sure you remove that user's email address from your block list, otherwise future emails from that user will get deleted. Click on the "Options" link in the upper right corner of Yahoo Mail. Then click the "Block Addresses" link and remove the email address that you want to unblock.