503
SMTP
503 Bad Sequence
The commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
Quand vous voyez ce code
When sending DATA before RCPT TO, or MAIL FROM before EHLO. The server enforces the correct SMTP command sequence and you have sent a command out of order.
Comment résoudre
Follow the proper SMTP command sequence: EHLO/HELO first, then MAIL FROM, then RCPT TO (one or more), then DATA. Reset with RSET if you need to start over.
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Guides
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
Scénarios de débogage
Spécifications
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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
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ARC (Authenticated Received Chain)
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Email Header Injection
SMTP Enhanced Status Codes
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