StopBouncingCommon SMTP errors to look up:
Understanding SMTP Response Codes
SMTP uses three-digit response codes grouped by category:
| Code range | Type | Action |
|---|---|---|
| 2xx | Success | No action required |
| 3xx | Intermediate | Further action required (e.g. send message body) |
| 4xx | Transient failure (soft bounce) | Retry delivery — temporary issue |
| 5xx | Permanent failure (hard bounce) | Do not retry — remove from list |
SMTP Error FAQ
What is the difference between a hard bounce and a soft bounce?
A hard bounce (5xx errors) indicates a permanent delivery failure — the email address does not exist, the domain has no mail servers, or the server permanently rejected the message. A soft bounce (4xx errors) is a temporary failure — the server is overloaded, the mailbox is full, or the connection timed out. Hard bounces must be suppressed immediately; soft bounces are typically retried by your email system.
What does SMTP error 550 mean?
Error 550 is the most common hard bounce code and usually means the recipient's email address does not exist. When combined with enhanced status code 5.1.1 (550 5.1.1), it specifically indicates the mailbox does not exist. Remove the address from your list immediately — continued sends to 550 addresses damage your sender reputation.
What causes SMTP error 554?
Error 554 means the transaction failed, usually due to the receiving server's spam filters or policy rules. Common causes include: your sending IP is on a blocklist, your SPF/DKIM/DMARC isn't set up correctly, or the message content triggered a spam filter. Check your IP reputation and verify your email authentication records.
Why does SMTP error 421 occur?
Error 421 is a temporary service unavailability error. The destination server is either overloaded, undergoing maintenance, or rate-limiting your connection. This is a soft bounce — your email system will typically retry delivery automatically. If it persists for more than 48 hours, investigate the specific server and message.