To be able to fully rely on the delivery of your scheduled emails, you should be using Mailbutler's Cloud Delivery. Only if you use Cloud Delivery, your emails will be delivered even if your computer is not connected to the internet at the time.
The huge disadvantage of MacOS Sonoma's (and macOS Ventura's) Local Delivery is that your computer needs to be connected to the internet at the time when your scheduled emails should be delivered.
Please note: Mailbutler always respects your privacy. If you use Mailbutler's Cloud Delivery, your emails are still being send from your email server. Your emails are not being saved or processed by Mailbutler. Mailbutler only informs your server to send the emails.
For detailed information on what makes Mailbutler's Smart Send Later better than macOS Sonoma's (and macOs Ventura's) Send Later, please see this blog post.
For detailed information on how to use Mailbutler's Smart Send Later feature, please see this article.