Does Mac Mail have an Auto Archive function as mail is being stored on Server using IMAP?
If your configured using IMAP on a Mac Mail might be building up on the server and Mac Mail has no auto archive function so you need to adopt a process of manually moving mail off the server.
Here is how to create local Folders in Mac Mail:
1. Create local folders on your computer within Mac Mail. To do this go to the bottom right corner of the column labeled "Mailboxes" on the left side of the main Mail window. There will be a plus sign there.
2. Click on the plus sign. You will be given three options, New Mailbox..., New Smart Mailbox... and Add RSS Feeds...
3. Select "New Mailbox" from the list. Under the Location drop down menu, select "On My Mac." Then give the folder a title that will help you remember what you have put in the folder.
4. You can create multiple folders if you choose to, by repeating one through three above.
5. If you would like to create sub-folders when you reach the Location drop down menu described in step three, simply select one of the folders located below the "On My Mac" option and you will be able to create a sub-folder within that folder. Just remember that a main folder with subfolders in it can not also hold emails. Once you have created a folder with subfolders the main folder can no longer contain emails. The subfolders that you place within the parent folder then contain the emails.
6. When you have created all of the necessary folders in the "On My Mac" section of your Mac Mail program. Click on the folder under your email account that contains the messages you would like to move. Select all the messages and drag and drop them into the folders you created under the "On My Mac" section . This will remove them from the email servers and save them permanently on your computer for you to look at when you need to.