A DNS error: the host server for the recipient's domain name cannot be found.
The email recipient's email address (firstname.lastname@example.org) is using a domain name that can not be found.
This is commonly due to misspelling in the recipient's email address such as (email@example.com instead of firstname.lastname@example.org). You can verify DNS records for the domain with tools such as 1)IntoDNS or 2)MXToolBox.
If the domain name is correct and discover-able, it's likely due to your mail server's network configurations or resolv.conf.
SMTP Error 550 5.1.2 Host unknown – host cannot be found