Quick answer: Email bounce-back messages tell you why a message could not be delivered or is delayed. A 5xx SMTP code usually describes a permanent failure for that attempt, whereas a 4xx code generally signals a temporary issue. Read the full diagnostic text: the number alone is rarely enough to identify the cause.
Common SMTP bounce codes
| Code | Often means | First check |
|---|---|---|
| 550 | Unavailable mailbox or policy rejection | Recipient address and diagnostic message |
| 554 | Rejection, often based on a policy | Authentication and spam-filter reason |
| 421 | Temporary server or rate-limiting problem | Automatic retry status |
| 450 / 451 | Temporary delivery problem | Mailbox status and retry details |
| 552 | Often a message size or storage limit | |
| 535 | Often an SMTP authentication failure | Outgoing login settings |
These are common interpretations, not universal meanings. Additional text and enhanced codes such as 5.1.1 or 5.7.1 help identify the actual issue.
1. Check which messages fail
Does the failure affect just one address or many? If one recipient fails, check spelling and whether that mailbox exists. If multiple unrelated recipients reject mail, investigate sender authentication, outgoing account configuration and provider policies.
2. Inspect the full delivery notification
Find the recipient, time, response text and sending service. A message saying “user unknown” needs a different solution from “unauthenticated mail rejected”. Beware of fraudulent bounce messages asking you to enter credentials on unfamiliar websites.
3. Check SPF, DKIM and DMARC
Receiving services can refuse email that fails authentication or domain alignment. SPF lists permitted sending systems; DKIM signs mail; DMARC checks alignment with the visible From address. Read our SPF, DKIM and DMARC guide for a careful setup walkthrough. Use records supplied by the services you actually send through.
4. Confirm SMTP settings
If an email client cannot send, check outgoing hostname, port, encryption and authentication against your provider’s instructions. WordPress contact forms can have separate sending issues; see our WordPress contact form email troubleshooting guide.
5. Check attachment sizes and sending limits
Large attachments may exceed your provider’s or recipient’s allowed message size. Sending a high volume of email can trigger rate limits. Confirm account restrictions and check whether mail storage or account quotas have been reached.
Should I resend immediately?
For temporary 4xx failures, the sending mail server normally retries. Manually resending repeatedly may cause duplicates or worsen limits. For a permanent 5xx refusal, address the underlying issue before retrying.
When to contact support
Send your provider the full error message, approximate time and affected domains through a secure support channel. Do not share passwords. Ask whether an outgoing restriction, authentication error or recipient-side rejection is responsible.
Frequently asked questions
Does error 550 mean the email address is invalid?
Not necessarily. Some servers use 550 for other permanent policy refusals.
Is error 421 permanent?
Usually not. It commonly represents a temporary condition.
Will SPF and DKIM guarantee my email arrives?
No. Authentication helps establish legitimacy but does not guarantee acceptance or inbox placement.
Do MX records fix outgoing mail?
Usually no. MX records route incoming mail for a domain; outbound sending can use a separate service.
Summary
Read the complete bounce text, identify whether one or many recipients are affected, and check sending configuration and authentication before changing DNS. The details of the rejection point to the correct fix.