Mail on Mac: Cannot save, insufficient storage

Re: Mail on Mac: Cannot save, insufficient storage

A Mac generally needs around 50 Gb free space in which to operate smoothly, and do all you ask it to.

You can check used and available storage space by going to:

Apple menu > System Settings > General > Storage

It is always wise to back up everything on your Mac first, before making space.

If needed: See How to back up your Mac - Apple Support

Then see how to: Free up storage space on Mac - Apple Support

Eg: says:

"The Storage information in System Settings shows how much storage space is used by various categories of files. As you follow the steps in this article to free up space, this storage information will update automatically.

• If you see a More Info button (i) next to a listed category, click it for more information and options for managing storage in that category. This is particularly useful for the Applications, Documents, Messages, Music Creation, Podcasts, Bin and TV categories. 
• The category named System Data (or Other) is a general category that measures the storage space used by all Apple and third-party files that don’t belong to any more-specific category.

Optimise storage space

Your Mac can optimise storage by using iCloud to automatically make more storage space available when needed.* It includes built-in utilities to help you quickly find files that are taking up space, and delete any that you longer need.

• macOS Ventura 13 or later: How to optimise storage space.
• Earlier versions of macOS: How to optimise storage space.

Move or delete files to free up storage space

• If you have another storage device, such as an external drive connected to your Mac, you can move files to that device. Media files such as photos and videos can use a lot of storage space, so you might want to move your Photos library or move your iMovie library, for example.
• Delete music, films, podcasts or other media, especially if it’s media that you can stream or download again as needed.
• Delete files in your Downloads folder. Open this folder from the Dock or by choosing Go > Downloads from the menu bar in the Finder.
• If you use the Mail app for email, choose Mailbox > Erase Junk Mail from the menu bar in Mail. Also choose Mailbox > Erase Deleted Items.
• If you use your Mac to back up your iPhone or iPad, you can delete old backups.
• Delete or uninstall apps that you no longer use.
• If your Mac is set up for multiple users, delete users who no longer need access to your Mac. Or ask them to follow the steps in this article after logging in to their user account.
• Check the Users folder on Macintosh HD for any undeleted data belonging to deleted users, as well as any shared filesthat you no longer need to share. 
• If you don't want to delete certain files, you may be able to save space by compressing them. Then expand them when you need to use them again.
• Empty your Bin. When you move a file to the Bin, its storage space won't become available until you empty the Bin."

If needed:

There is more info. in that linked "Free up Storage Space ... " article at the top.