How to fix error while moving messages in Apple Mail

A common problem with Apple Mail (also often called Mac Mail) is the "error occurred while moving messages to mailbox (null)" error. This problem occurs when folders for the Drafts, Sent, Junk, or Trash mailbox are not assigned. This error most commonly occurs when deleting messages and the Trash mail box has not been assigned, but happens whenever messages are moved without the folders being assigned. Here's a screenshot of the error:

In order to use these instructions, you will need to have added an email account into the Apple Mail client.

Step 1. Login to the Apple Mail Client
Step 2. Scroll down to your newly added mail account. Click on the drop-arrow to see the Drafts, Sent, Junk, and Trash folders, then click on the Drafts folder to highlight it
Step 3. In the Menu at the top of the screen, click on Mailbox, then scroll down to Use This Mailbox As, then click on the Drafts Mailbox. This will assign the Drafts folder in the client to the corresponding server folder.
Step 4. Repeat this action for all of your folders (Sent, Junk, and Trash), and make sure to click on the respective folder assignment. As you add these folders, you will see the email account name added for each option.