I am wondering how email servers usually treat an email that has no real email domain from which it originated in comparison to an email that has a real email domain such as firstname.lastname@example.org. How do email servers treat such emails when they are received. For example, some programs, such as nagios, generates emails through which users can be notified of server problems. The email looks like this when it is received: User for Nagios [email@example.com]. Although, you can not send an email back to this address becuase the domain does not exist. My question is how most email servers would treat such emails (perhaps as spam) if they were sent on a regular basis.
Thanks for the help ahead of time.
[EDIT: my bad if this post does not belong here]