Here is how to stop the app from popping up: Some users complain that their Mac email app keeps popping up on split screens. How to Stop Mail App From Popping Up on Mac
Manage your mailbox overloaded with unwanted emails more efficiently Try Clean Email for Free How to Change Default Mail App on Mac Underneath, you’ll find out how much space attachments are occupying, select and delete all attachments you no longer need. Click the Storage tab and then select Manage -> click on Mail in the left-hand sidebar. Click on the “Apple menu” and select About this Mac.
With the latest macOS, you can also use the optimize feature to delete attachments.This will sort out all messages with attachments. If you are having trouble finding emails with attachment, simply click the View menu, and then select View by attachments.
Alternatively, you can simply delete the email, and proceed to your download directory in the library on your Mac to delete downloaded attachments. To delete, simply navigate to messages with attachments, click Menu and select Remove attachment.
Locate any folders bearing the following names, and drag them to the Trash. Then, hold an Option key while selecting the Finder's Go menu.
A popup window may open when you do that, pleading for you to reconsider.