Troubleshooting Mail Delivery Sub-System Mailer Daemon
About Mailer Daemon and Returned Emails.
Usually when an email is returned with this error message it means that the email address is entered incorrectly - possibly spelled wrong or with a .net instead of a .com - or it could mean that the account that you are sending to no longer exists. The mailer-daemon message is actually from your own email server, not from the intended recipient, and is informing you that the message is undeliverable. It is sometimes the case that the intended recipient's email inbox is full as so the message is returned.
Possible Routing Issues and Internet Access Restrictions
Another possible cause of the mailer daemon error message return might be a routing problem. For example, it is possible that the organization from which you are sending the email might have some restrictions on its email or Internet access. For example, many businesses and organizations create limits on their Internet access that might be interfering with your emails. Some companies impose limits like these to restrict the amount of time that their employees can spend on non-work related Internet use. Also, some public entities limit Internet access to sites deemed inappropriate for reasons of propriety, safety or liability. To get to the bottom of your email errors you can try sending an email to the same address from a different location and see if it goes through. If it does, it means that the problem was with the location you were sending from.
Restricted Domain Names
Another possibility is that your domain name could be blacklisted, in which case no messages from that address will be admitted. Blacklists are created to restrict emails from the addresses and domains of known spammers. Other organizations are sometimes blacklisted due to security restrictions, if they are known to be vulnerable to Spyware and other dangerous malware programs. It is possible that your email address has been blocked by the address that you are attempting to send to.
Try Sending from Different Locations
The hospital where you work might have some Internet or email restrictions that are preventing your messages from getting through, or there could be a blacklist from the hospital against any government agency or public office for reasons of security. The only way to get to the bottom of your problem is to try the same addresses from a different location, or to different addresses from the same location and see what works.