This may be caused by permissions. On Mojave Mail is blocked from apps by default. In Bash you will see this by default:

$ cd ~/Library/Containers/com.apple.mail

$ ls

ls: cannot open directory '.': Operation not permitted

And defaults read -app Mail will only show these options:

{

BundleCompatibilityVersion = 3;

EnableBundles = 1;

NSNavLastRootDirectory = "...";

NSNavPanelExpandedSizeForOpenMode = "{720, 448}";

}

You need to add Bash (and maybe others like iTerm, Terminal, etc) to the Full Disk Access list. When you click +, use ⇧+ ⌘ + G to open the Go to folder dialog so you can pick items like Bash. You need to pick the exact one you are using. I am using MacPorts so everything is in /opt/local/bin for me.

Once you've done this, start a new shell (already running shells will not work) and type defaults read -app Mailand it will show all settings. It does not work for me in tmux (not sure why yet). Run this command in a bare shell.

$ defaults read -app Mail | fgrep DisableSendAnimations

DisableSendAnimations = 1;