451
SMTP
451 Local Error
The requested action was aborted due to a local error in processing. The server encountered an internal issue that prevented it from completing the command.
Cuándo verás este código
When the receiving server hits an internal error such as a database failure, DNS lookup timeout, or anti-spam filter processing error.
Cómo solucionarlo
Retry the message later. If you control the server, check its logs for the specific error — common causes include DNS resolution failures, disk I/O errors, or database connectivity issues.
Especificación
Comparar con
Guías
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Diagnosing Email Bounces: Soft (4xx) vs Hard (5xx)
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DMARC Reporting: Analyzing Aggregate and Forensic Reports
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Email Feedback Loops (FBL): Processing Spam Complaints
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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
RFC 959: FTP Protocol Deep Dive
Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure
Protocol Deep Dives
Escenarios de depuración
Especificaciones
Guías de decisión
Glosario
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)