In Options > Cron Details, how is your CRON set up? WP CRON or manual CRON using the link they provide?
You might just be able to copy that CRON job URL and paste it in a browser to kick it. If you’re using WP CRON, how often is your WP CRON called? Just when people visit the website? Or do you have CRON scheduled somehow on your server so it’s called on a more regular basis?
For WP CRON, the plugin has 3 hooks, but generally only the hourly and monthly are called. So if you depend on WP CRON, the shortest time you’ll wait for mail to send is whenever that hourly cron hook gets called.
I use WP CRON but mine is called every minute from the server’s crontab using WP CLI so it’s all pretty instant. It also insure that anything scheduled happens on time. Still, it’s limited to that hourly hook the plugin sets up. For us, waiting up to an hour for messages to go is good enough.
