SMTP
556 Domain Does Not Accept Mail
The destination domain does not accept mail and no forwarding address is available. The domain's DNS configuration (null MX record) explicitly indicates it does not receive email.
Cuándo verás este código
When sending to a domain that has published a null MX record (RFC 7505) in DNS, explicitly declaring that it does not accept any email messages.
Cómo solucionarlo
Verify the domain's MX records — a null MX (priority 0, empty host) means the domain intentionally rejects all mail. Contact the recipient through an alternative channel.