556
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.
このコードが表示される場合
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.
解決方法
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.
比較対象
ガイド
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Debugging & Troubleshooting
SMTP Reply Codes: What Your Email Server Is Telling You
Email Delivery
Improving Email Deliverability: SPF, DKIM, and DMARC Setup
Email Delivery
Diagnosing Email Bounces: Soft (4xx) vs Hard (5xx)
Email Delivery
SMTP Authentication Methods Explained
Email Delivery
Email Header Analysis: Tracing Delivery Path
Email Delivery
Bulk Email Delivery: Avoiding the Spam Folder
Email Delivery
DMARC Reporting: Analyzing Aggregate and Forensic Reports
Email Delivery
IP Warming for Email: Building Sender Reputation from Scratch
Email Delivery
Email Feedback Loops (FBL): Processing Spam Complaints
Email Delivery
Transactional Email Best Practices: Receipts, Alerts, and Password Resets
Email Delivery
DNS Record Types Explained: A, AAAA, CNAME, MX, TXT, and Beyond
DNS & Networking
TCP Three-Way Handshake and Connection Lifecycle
DNS & Networking
RFC 5321: SMTP Protocol Deep Dive
Protocol Deep Dives
RFC 959: FTP Protocol Deep Dive
Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure
Protocol Deep Dives
用語集
Status Code
Media Type (MIME Type)
TLS Handshake
TLS Certificate (SSL Certificate)
Cipher Suite
Authentication
TCP/IP
Three-Way Handshake
Port Number
Firewall
IPv6
Exponential Backoff
Error Code Mapping
Poison Message
Request-Response Model
RFC (Request for Comments)
IANA (Internet Assigned Numbers Authority)
Message Framing
Connection-Oriented
Text vs Binary Protocol
MX Record
PTR Record
SMTP Relay
SPF, DKIM, and DMARC
Bounce Rate (Email)
Email Deliverability
MTA (Mail Transfer Agent)
SMTP Pipelining
Greylisting
Backscatter
Envelope vs Header
STARTTLS
SMTP Authentication (SMTP AUTH)
Open Relay
Email Queue
MIME Multipart
ARC (Authenticated Received Chain)
BIMI (Brand Indicators for Message Identification)
DSN (Delivery Status Notification)
Email Header Injection
SMTP Enhanced Status Codes
MDN (Message Disposition Notification)
MIME Type
TLS Versions
TLS 1.3
Certificate Authority (CA)
Forward Secrecy (PFS)
SNI (Server Name Indication)
TLS Session Resumption
Key Exchange
PKI (Public Key Infrastructure)