SMTP Errors (Protocol Response Codes)

Complete reference guide to SMTP errors (protocol response codes). Learn about all SMTP reply codes, their meanings, and how to handle them.

SMTP Error 421: Service Not Available

421-service-not-available

SMTP Error 450: Mailbox Unavailable

450-mailbox-unavailable

SMTP Error 451: Local Error in Processing

451-local-error

SMTP Error 452: Insufficient System Storage

452-insufficient-storage

SMTP Error 453: You Have No Mail

SMTP Error 454: TLS Not Available

454-tls-not-available

SMTP Error 455: Server Unable to Process

455-server-unable-to-process

SMTP Error 500: Syntax Error, Command Unrecognized

500-syntax-error

SMTP Error 501: Syntax Error in Parameters or Arguments

501-syntax-error-parameters

SMTP Error 502: Command Not Implemented

502-command-not-implemented

SMTP Error 503: Bad Sequence of Commands

503-bad-sequence

SMTP Error 504: Command Parameter Not Implemented

504-parameter-not-implemented

SMTP Error 521: Does Not Accept Mail

521-does-not-accept-mail

SMTP Error 530: Must Issue a STARTTLS Command First

530-starttls-required

SMTP Error 550: Mailbox Unavailable (Permanent)

550-mailbox-unavailable-permanent

SMTP Error 551: User Not Local

551-user-not-local

SMTP Error 552: Exceeded Storage Allocation

552-exceeded-storage

SMTP Error 553: Mailbox Name Not Allowed

553-mailbox-name-not-allowed

SMTP Error 554: Transaction Failed

554-transaction-failed

SMTP Error 555: Parameters Not Recognized or Not Implemented

555-parameters-not-recognized