The module doesn't allow you to send anything if the start/end date is in the past.
If you select today in the module, then then the notification is sent now, otherwise it's scheduled.
Emails are sent by Bcc if the module property is set.
Notifications are only sent to activated users who are not deleted.
Email notifications only go out if user preferences are set.
If there are less than 20 users receiving the notification then the single email is sent to everyone by framework.
If there are more than 20 users receiving the notification then framework creates an email event for each user, handled by the standard module. Each event sends a single email to a single user, with no delay in between emails.
Every time an email is being sent a new connection to the mail server is created. If all users are not receiving the notifications, it is most likely due to the email system detecting the portal as spam and blocking it. Rate limits and possibly getting the portal white listed should be looked in to my the IT team. Bumping the max connections will work but we find that some connection levels are lowered on purpose to prevent abusing the system.