521
SMTP
521 Host Does Not Accept Mail
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
このコードが表示される場合
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
解決方法
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.
比較
比較対象
ガイド
Debugging SMTP Delivery Failures: 4xx Temporary vs 5xx Permanent
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)