I've searched through the plist file (/Users/<user>/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist) and can't seem to find where I can disable 'Automatically download all attachments' function in Accounts > Advanced. I've done a before and after capture and a delta in what data is written, and I can't find the correct settings to change this.
I've found that our users that have Mail.app configured are pulling down email from Google SPAM folder which Google throws questionable emails into. So the SPAM with attachments then show up in our Sophos console.
I'd prefer if the download automatically goes away so I can not have to risk anything bad happening to my users.
