How many times have you tried to open a mail message but the mail just freezes because it’s building the previews for the message attachments?
By default the Mail app provides a preview of any attachments it can read, however, you can turn it off by running a simple Terminal command:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
Then, quit Mail and relaunch it.
Now when you view a message, all attachments will be shown as icons only.
To restore it back to defaults, just run the this command:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool no