421
SMTP
421 Service Not Available
The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
Quando você vê este código
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Como corrigir
Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.
Especificação
Comparações
Comparar com
Guias
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DMARC Reporting: Analyzing Aggregate and Forensic Reports
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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
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Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure
Protocol Deep Dives
Cenários de depuração
Especificações
Guias de decisão
Glossário
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)