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.
Cuándo verás este código
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.
Cómo solucionarlo
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.
Especificación
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Guías
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Protocol Deep Dives
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Protocol Deep Dives
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)