This type of error code generally means that your SMTP Server is unable to deliver the communication to the intended recipient/s because his/her mailbox/address does not exist and is a hard bounce. 

For more general information about hard and soft bounce codes, consult our Email Resource Center: 
https://docs.blackbaud.com/email-resource-center/faqs/processing#what-is-a-hard-undeliverable-email-hard-bounce