Understanding Email Bounce-Back Messages and What They Mean

A bounce-back is the automated reply you get when an email cannot be delivered. It looks alarming, but it is really just the mail system telling you why a message did not arrive — and buried in it is usually a clear clue about how to fix things.

Hard bounces vs soft bounces

• Hard bounce — a permanent failure. The address does not exist, or the domain is wrong. These will never succeed; correct the address.
• Soft bounce — a temporary problem. The mailbox is full, the server is briefly down, or the message is too large. These often succeed on a retry.

Reading the error code

Bounce messages include a status code. A few common ones:

• 550 — mailbox unavailable / user unknown. The address is wrong or no longer exists.
• 552 — mailbox full or message too large. The recipient needs to clear space, or you need to shrink an attachment.
• 421 / 451 — try again later. A temporary server issue; the message will usually retry on its own.
• 550 relaying denied. A sending-side authentication issue — see fixing the relaying denied error.

When your mail keeps bouncing to everyone

If messages bounce no matter who you write to, the problem is on your side, not theirs. The usual culprits are wrong outgoing settings, a blocked IP reputation, or missing authentication. Setting up SPF, DKIM and DMARC resolves a surprising number of these, and our guide on improving deliverability goes deeper.

Frequently asked questions

The address is definitely correct, but it still hard-bounces.

Occasionally a recipient's server misreports a full or blocked mailbox as "user unknown". Ask the recipient to check with their provider, and confirm you are not on a blocklist by reviewing your email headers.

Do soft bounces need action from me?

Usually not — the server retries automatically for a while. If a soft bounce persists for a day or more, treat it like a hard bounce and investigate.

How do I stop bounces from hurting my sending reputation?

Keep your recipient lists clean, remove addresses that hard-bounce, and authenticate your domain. Consistent bouncing to dead addresses is exactly what damages reputation over time.