Infrastructure
What is SMTP (Simple Mail Transfer Protocol) Error Code?
The numeric reply code an SMTP server returns to indicate whether a message was accepted, temporarily deferred, or permanently rejected.
SMTP reply codes are three-digit numbers where 2xx means success, 4xx means a temporary failure that the sender should retry, and 5xx means a permanent failure that should not be retried. They are often paired with an RFC 3463 enhanced status code (for example 5.1.1 for an unknown recipient) and a human-readable text string. The receiving MTA returns these during the SMTP conversation, and the sending system maps them to soft or hard bounces.
Why it matters for cold email
Correctly interpreting codes is what separates a recoverable 4xx greylisting or rate-limit deferral from a 5xx that means the address is dead and must be suppressed immediately. Treating permanent 5xx failures as retryable inflates bounce rate and damages sender reputation, while honoring them keeps a cold list clean.
Related terms
Stop guessing at deliverability.
Scheduler Zero handles authentication, warmup, and sender reputation so your cold email lands in the inbox, not the spam folder.