iPhone 12 mini

You are going to have to do like I said earlier and like the message dialog states. You have too much there. You should access your mail from the web access for that account for now until you have cleared content from your iPhone.

Is it that you cannot open mail, or is it that your storage is full and you cannot receive any additional mail? What have you done to reduce the used storage on your device to you have more? An iPhone should have at a minimum, 10% of its total storage available as free. The only way to increase storage is to remove content. When you go to Settings>General>iPhone Storage, it will provide you with Recommendations for reducing storage use and also let you know how your storage is being used so you can consider deleting some content that way as well.