1. See how much Mail is really using
Open System Settings → General → Storage and look for Mail in the category list. The figure covers the local copies of your messages and the attachments Mail has downloaded. A disk scanner shows the same thing from the file side: the Mail folder inside your Library and Mail’s container, which also holds a Mail Downloads folder for attachments you opened without saving.
Apple notes that attachments you save are put in the Downloads folder by default, or in a folder you choose under Mail → Settings → General. So the same PDF can exist three times: inside the message, in Mail Downloads, and in Downloads. The steps below shrink each copy through Mail rather than around it.
2. Stop Mail downloading every attachment
Choose Mail → Settings → Accounts, select the account, and open Account Information. The Download Attachments menu offers All, Recent and None. Apple describes Recent as attachments received within the past 15 months and None as no automatic download. Mail always downloads media attachments such as images, PDFs, video and audio; the menu applies to other types like spreadsheets and zipped files.
Recent is the sensible setting for a laptop with a small SSD. Messages and attachments stay on the server for iCloud and other IMAP accounts, so Mail fetches an older attachment when you open it. Repeat the setting for each account; it is per account, not global.
3. Remove attachments from messages you keep
Select a message and choose Message → Remove Attachments. Apple’s guide says the message stays in the mailbox with a note that attachments were removed. The warning matters: for IMAP accounts the attachment is deleted from the mail server permanently, so save anything you still need first, then remove it.
Work through the biggest messages rather than everything. In Mail, sort a mailbox by size or search for messages with attachments, save the files you want to keep to a folder you control, and remove the rest. Do not run this across a whole mailbox in one pass; the deletion is permanent on the server.
4. Erase deleted and junk messages
Deleted messages are not gone until they are erased. Choose Mailbox → Erase Deleted Items and pick an account, or Control-click a Trash mailbox in the sidebar and choose Erase Deleted Items. Apple’s storage guidance lists this step too. The Junk mailbox has its own Erase Junk Mail command.
To make it routine, open Mail → Settings → Accounts, select the account and click Mailbox Behaviors. The Trash Mailbox menu sets where deleted messages are kept, and the erase option sets when they are removed for good. POP accounts have separate retention settings under Advanced; check them before assuming an old message is safe on the server.
5. What not to delete by hand
Do not delete folders inside ~/Library/Mail or Mail’s container in Finder. They hold your mailboxes and the index Mail uses to search them. Removing them can lose messages that were never on a server, and for IMAP accounts Mail simply downloads everything again. Use the settings above, and treat those folders as review-only when a scanner lists them.
If Mail is still large afterwards, the size is the messages themselves. Create mailboxes and use Rules under Mail → Settings to sort mail automatically, export anything you need offline with File → Save As or File → Export as PDF, and delete the rest. A local scan can confirm where the space went and whether the Downloads folder holds saved attachments you have already dealt with.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.