The short answer
No. Your data. Shrink it from its app; don’t delete the folder.
What is it?
Apple Mail’s local store: every mailbox, message and downloaded attachment for all your accounts, plus On My Mac mailboxes and Mail’s database. It commonly reaches several GB, and tens of GB for large mailboxes.
Your local copy of every mailbox. Never deleted by MoreRoom.
What happens if you delete it?
For iCloud, IMAP and Exchange accounts Mail downloads messages again, which takes time; On My Mac mailboxes and anything stored only locally are lost for good.
How to remove it properly
Do not delete it in Finder. In Mail › Settings › Accounts, select an account and set Download Attachments to Recent or None; delete old mail and erase deleted items. For large messages, select them and choose Message › Remove Attachments. Optimize Storage in System Settings › General › Storage also keeps only recent attachments.
From the app itself: Mail: select large messages and choose Message > Remove Attachments, or delete old mail.
Before you delete it
On iCloud and IMAP accounts, removing attachments or deleting messages also changes the copy on the server and on your other devices.
How MoreRoom helps
MoreRoom recognises this as your data and never suggests deleting it. It shows how much space it takes, so you can shrink it from its own app.
Frequently asked questions
Can I delete the ~/Library/Mail folder on Mac?
No. Your data. Shrink it from its app; don’t delete the folder. For iCloud, IMAP and Exchange accounts Mail downloads messages again, which takes time; On My Mac mailboxes and anything stored only locally are lost for good.
Will Mail mailboxes come back after I delete it?
No. For iCloud, IMAP and Exchange accounts Mail downloads messages again, which takes time; On My Mac mailboxes and anything stored only locally are lost for good.
How do I remove Mail mailboxes properly?
Do not delete it in Finder. In Mail › Settings › Accounts, select an account and set Download Attachments to Recent or None; delete old mail and erase deleted items. For large messages, select them and choose Message › Remove Attachments. Optimize Storage in System Settings › General › Storage also keeps only recent attachments.