4xx and 5xx email errors are grouped into two broad classes. A 4xx response usually points to a temporary problem. A 5xx response usually means that the server rejected the request and action is needed. The exact meaning depends on the protocol and the full error message. SMTP error codes are three-digit responses returned when a mail server cannot complete delivery. Learn more about SMTP error codes.
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How 4xx and 5xx errors differ
A 4xx response normally indicates a temporary delivery or connection failure. Retrying later may succeed if the server becomes available or the temporary condition clears. A 5xx response normally indicates a permanent rejection of the current request. Repeating the same request without changing anything is unlikely to solve the problem.
For email, the right next step depends on where the error appears:
- SMTP: the error is usually related to sending or delivery to a recipient.
- IMAP: the error appears while a mail app connects to the mailbox and reads or synchronises messages.
- POP3: the error appears while a mail app connects to the mailbox and downloads messages.
The three protocols can therefore show similar error classes but point to different actions. An SMTP error may require a recipient or delivery check. An IMAP or POP3 error may require a check of the mail app settings or account access.
SMTP 4xx errors: temporary sending or delivery problems
An SMTP 4xx response usually means that delivery cannot be completed at that moment. The receiving or sending mail server may be temporarily unavailable, or the message may need to be retried later. A temporary response does not by itself prove that the recipient address is wrong or that the mailbox is permanently unavailable.
When an SMTP 4xx error appears, use this order:
- Wait briefly and retry sending the message.
- Check that the recipient address is entered correctly.
- Read the complete server message, not only the three-digit code.
- If repeated attempts continue to fail, contact support with the full error text and the time of the failed delivery.
A retry is appropriate when the error is temporary. Sending the same message repeatedly in a short period is not a substitute for checking the exact response.
SMTP 5xx errors: rejected delivery
An SMTP 5xx response usually means that the server has rejected the request and will not complete it as submitted. Error 550 is a clear example. It means that the recipient’s mail server rejected the message and refused delivery. Unlike a temporary delivery error, a 550 error usually requires action before the message can be delivered. See the mybox explanation of Error 550.
For an SMTP 5xx error:
- Review the recipient address, including the domain after the
@symbol. - Read the rejection text for an indication of whether the address, account, or server policy caused the refusal.
- Do not rely on repeated retries if the same 5xx response returns each time.
- Contact support when the address is correct and the rejection continues, providing the complete SMTP response.
The receiving mail server controls the final acceptance of a message. A 5xx response can therefore require action by the recipient’s mail provider or by the administrator of the sending account.
IMAP 4xx and 5xx errors in a mail app
IMAP errors affect the connection between the mail app and the mailbox. They commonly appear when the app cannot sign in, read messages, or synchronise the mailbox. In this situation, the message may not have been sent or rejected by a recipient. The problem is the connection to the mailbox.
A temporary IMAP response should be treated as a connection problem first. Wait and retry the connection. If the error continues, check the account settings in the mail app and confirm that the saved email address and password are current. The mybox troubleshooting steps for messages that stop appearing begin with checking the email client settings. See the mail client troubleshooting steps.
A permanent IMAP response usually needs a settings or account check rather than repeated retries. Review the account details, then contact support if the credentials are correct and the mail app still cannot access the mailbox.
POP3 4xx and 5xx errors in a mail app
POP3 errors affect the connection used by a mail app to download messages. A temporary POP3 response may clear after a later retry. A permanent response usually requires action in the account configuration or a review of mailbox access.
Start with the same practical checks used for other mail client connection problems:
- Check the email address saved in the mail app.
- Check the password and update it if the account credentials have changed.
- Review the mail app’s account settings.
- Retry after correcting any account detail that does not match the mailbox.
- Contact support if the details are correct but the POP3 connection continues to fail.
A POP3 connection error does not describe the same event as an SMTP delivery rejection. POP3 concerns downloading messages from the mailbox, while SMTP errors concern sending or delivering a message.
Quick comparison by protocol
| Protocol | What the error usually affects | First action |
|---|---|---|
| SMTP | Sending or delivery to a recipient | Retry a 4xx response; check the recipient address and rejection text for a 5xx response |
| IMAP | Reading or synchronising the mailbox | Retry a temporary error; check mail app settings and credentials for a persistent error |
| POP3 | Downloading messages from the mailbox | Retry a temporary error; check mail app settings and credentials for a persistent error |
When to contact support
Contact support when a 5xx error continues after checking the recipient address or account details, when a 4xx error persists after reasonable retries, or when the complete message does not clearly identify the cause. Include the protocol, the full error text, and the time it occurred. These details help distinguish a delivery rejection from a mail app connection problem.