Disclosure up front: I make Legroom, a Mac disk-space app. Nothing below needs it. Every step uses Mail itself, and the Download Attachments and Remove Attachments behaviour comes from Apple's Mail guide.
The short answer
Usually attachments. Mail stores a full copy of each message on your Mac, and a few messages with photos or video can outweigh thousands of plain ones. The way to shrink it is to change what Mail downloads and remove attachments from inside Mail. Do not delete files in the Mail folder with Finder.
See how big Mail is
Mail keeps its data in two places in your home folder: ~/Library/Mail (the messages, attachments and search indexes) and ~/Library/Containers/com.apple.mail (working data). In Finder, choose Go, then Go to Folder, paste the path and use File, then Get Info. In Terminal, du -sh ~/Library/Mail prints the total, and Terminal may need Full Disk Access to read it.
On my Mac, on October 26, 2026, the first was 1.6 GB and the second 13 MB. A report this week on r/MacOS described 163 GB of 1 TB used by Mail, so the range is wide.
Check the account type first
If the account is IMAP (iCloud, Gmail and Outlook are), the mail server keeps everything and your Mac holds a cache, so shrinking the cache is low risk. If it is POP, your Mac may hold the only copy of some messages. Back those up before you remove anything.
Steps inside Mail
- Find the big messages. In a mailbox, choose View, then Sort By, then Size. The largest messages come first. Delete the ones you do not need, or save their attachments to a folder first.
- Change what Mail downloads. Choose Mail, then Settings, click Accounts, pick an account and click Account Information. Apple's page says the Download Attachments menu has three choices: All, Recent (received in the past 15 months) and None. It applies to types such as spreadsheets and zip files. Apple says media attachments, meaning images, PDFs, video and audio, are always downloaded, so this setting alone will not stop photos from filling the folder.
- Remove attachments from old messages. Select messages, then choose Message, then Remove Attachments. The message stays. This is permanent, and for IMAP accounts Apple says the attachments are also deleted from the mail server and cannot be retrieved. Save anything you want to keep first.
- Empty Deleted Items and Junk. The Mailbox menu has Erase Deleted Items and Erase Junk Mail. Deleted mail keeps using disk until you do. I did not check Apple's page for these two, so confirm the names in your own menu.
The numbers can take a while to drop, and the Storage pane in System Settings may lag behind. Quit and reopen Mail, then measure again.
What not to do
Do not select files inside ~/Library/Mail and delete them in Finder. Mail tracks messages, mailbox state and search indexes together, so removing pieces by hand can leave it confused.
What I could not confirm
Two people in that r/MacOS thread said Mail's logs, under ~/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/, grew to 10 GB or more. That folder does not exist on my Mac, and I found no second source, so I cannot say it is a common cause. If you see a large Logs folder there, treat it as an unusual case and tell me.
I also did not test these steps on a 100 GB mailbox. They come from Apple's Mail guide and from reports in the r/MacOS thread, and I did not run them on my own Mac.
Related guides
- Why is System Data so big on my Mac?: what fills the System Data category and which causes are safe to remove.
- What AI tools leave on your Mac, and what is safe to delete: measured sizes, including Messages attachments.
- Best disk space analyzer and cleaner for Mac (2026): maps, cleaners and what is safe to delete.
- Sent to the wrong person: what Apple Mail can and cannot catch: another Apple Mail guide.
- Legroom, all products and all guides.