SMTP Error 451 4.1.0

I'm adressing to you guys because I'm having a problem when sending an e-mail to a specific adress, I'm getting this message :

Diagnostic-Code: smtp; 451 4.1.0 Recipient unknown

I looked for what it could be but couldn't find what the 4.1.0 code stands for . When I browse about the "Recipient unknown" the answers I find are about the SMTP error 500 which looks pretty confusing to me. At this point i'm trying to understand what the 4.1.0 code means for SMTP error 451 , and try to guess why I'm having this error.

Any help here would be higly appreciated !