What this means in practice
This avoids accepting mail that will later bounce to an address that may not belong to the sender.
Early refusal also protects delivery reputation for other customers sharing the same sending infrastructure.
Practical guidance
An unknown recipient is rejected during the SMTP conversation, before the message is accepted. This is a deliberate mail-handling choice. If a server accepted a message for an address that does not exist and sent a later bounce, that rejection could go to an unrelated or forged sender. Such backscatter can harm the reputation of the sending IP and affect delivery for other customers.
For a legitimate sender, the refusal indicates that the recipient address was not recognized at delivery time. Verify the spelling and confirm that the mailbox or alias exists before retrying. If the address should be valid, review its mailbox configuration instead of repeatedly sending the same message. Refusing unknown recipients early gives the sending system a clear result and avoids generating a second message to an address that may not belong to the original sender.