What is 550 rule and who is daemon

Please read this articles and answers, and check if they help.

https://www.experts-exchange.com/questions/23256719/5-5-0-smtp-550-Rule-imposed-mailbox-access-for.html

https://www.experts-exchange.com/questions/28550738/550-Rule-imposed-mailbox-access-for-abc-a-com-au-refused-user-invalid.html

https://www.experts-exchange.com/questions/24612316/550-Rule-imposed-mailbox-access.html

There will be more info in the bounce message that will tell you exactly why it was returned. A 550 is a hard bounce and usually means the recipient mailbox is not found, or you are being blocked by the remote server.

daemon is a common term for an automated process or job.

550 is a hard bounce of the email. As mention in other answer, you can find all the required explanation in the NDR that you will receive after the fail. There can be many reasons for this: blocked, hard fail of delivery from your server, mailbox not found/invalid etc.

Deamon: Any process that runs on the server (linux) for the purpose of automation is called a deamon. It can be a part of a process or an independent automation functionality.