The short answer
Yes. Rebuilt or re-downloaded automatically when needed.
What is it?
When you open an attachment from Apple Mail, a copy is saved in this Mail Downloads folder so the app that opens it has a file to work with. Over the years it can reach several GB.
Copies made when you open an attachment. The original stays in the message.
What happens if you delete it?
The originals stay in the messages, so any attachment can be opened again; Mail makes a new copy when you do.
How to remove it properly
Quit Mail, then move the contents of ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads to the Trash.
Quit the app that uses it, then move it to the Trash. Empty the Trash when you’re sure.
Before you delete it
If you edited an opened attachment and saved it in place, that edited version exists only here: save it somewhere else first.
How MoreRoom helps
MoreRoom recognises this item, shows its real size and when it was last used, and labels it “Safe to remove”. In the free version you can move it to the Trash in one click, with Undo; Pro collects every safe item at once.
Frequently asked questions
Is it safe to delete Mail Downloads on Mac?
Yes. Rebuilt or re-downloaded automatically when needed. The originals stay in the messages, so any attachment can be opened again; Mail makes a new copy when you do.
Will Mail attachments opened come back after I delete it?
Yes. The originals stay in the messages, so any attachment can be opened again; Mail makes a new copy when you do.
How do I remove Mail attachments opened properly?
Quit Mail, then move the contents of ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads to the Trash.