Email error codes look cryptic, but most fall into a small number of categories. Here’s what the common ones actually mean.
How these codes are structured
Most email bounce and error codes follow a standard numbering system used broadly across mail servers, not just one provider. The first digit tells you the general category of the problem.
Temporary (4xx) vs. permanent (5xx) errors
Codes starting with 4 are generally temporary — the receiving server is asking the sender to try again later, often because of a full mailbox or a brief outage. Codes starting with 5 are permanent failures, meaning the message won’t be delivered no matter how many times it’s retried, usually due to an invalid address or a policy rejection.
Common codes and what they mean
- 500 (unknown user): The username or address doesn’t match any existing account — often a typo in the recipient’s address.
- 550 (non-existent user / mailbox unavailable): Similar to 500, but can also mean an account was deleted or never fully set up.
- 421 (service unavailable / too many recipients): The server is temporarily rejecting the connection, often due to sending to too many recipients at once or a temporary rate limit.
- 552 (storage exceeded): The recipient’s mailbox is full and can’t accept new mail until space is freed.
- 554 (transaction failed): A general catch-all failure, often related to spam filtering rejecting the message outright.
What to do when you see one
For a permanent (5xx) error, double-check the recipient’s address for typos before resending. For a temporary (4xx) error, waiting and retrying is often enough, since the issue is usually on the receiving server’s end rather than something you can fix directly.
Frequently asked questions
What’s the difference between a 4xx and 5xx error?
4xx codes indicate a temporary problem worth retrying later; 5xx codes indicate a permanent failure that won’t resolve itself without a change, like fixing a typo in the address.
Why did I get a ‘mailbox full’ error sending to someone else?
This means the recipient’s own mailbox has reached its storage limit and can’t accept new mail until they free up space — it’s not something you can fix from your end.
Are these error codes the same across all email providers?
The general numbering system (4xx temporary, 5xx permanent) is a widely used standard, though the exact wording of the message can vary by provider.