SMTP
550 Mailbox Not Found
The requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
このコードが表示された場合
When sending to an email address that does not exist on the destination server, or when the server's policy rejects your message (e.g., anti-spam, sender verification failure).
修正方法
Verify the recipient's email address for typos. If the address is correct, the mailbox may have been deleted or your domain may be blocked — check your sender reputation and SPF/DKIM/DMARC records.