When you receive a message from the mailer daemon, this is an indication something has gone wrong with an email service. The mailer daemon is the computer program responsible for managing your email service, so it alerts you to any errors. If you continue receiving messages, verify that the email address you are sending email to is correct, as you may be sending email to an invalid address.
- Check the target email address
If the email address you typed is incorrect, the mailer daemon from the target server sends back a response with a message similar to "550 email@example.com User unknown." This indicates that the error exists before the @ symbol. Ensure that the username is correct, matching all the letters, numbers and capitalization of the email address you intend.
- Ensure the target mail server is correct
Another potential error may be the server name, which follows the @ symbol. If the message from the mailer daemon reveals a message similar to "550 firstname.lastname@example.org Host unknown (Name server: exampleserver.net: host not found)," this means that your email service cannot find the target email service.
- Look for a warning message
Sometimes, the mailer daemon sends a message as a warning that something may have gone wrong or as a warning against potential abuse. This does not necessarily mean that your message did not go through, but contact your recipient to make sure. You may also try to get in touch with the administrator of the email server, which is the web address following the @ symbol, such as exampleserver.net.