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;
