Soft bounce: temporary email delivery failure

Soft bounce is a temporary refusal to deliver an email. The recipient’s mail server signals that delivery is not possible right now but may succeed later. Unlike a hard bounce, a soft bounce does not mean the address is invalid.

Common causes

• Mailbox full. The recipient has hit their storage quota. Once they delete some messages, or an admin raises the limit, delivery can proceed.
• Server temporarily unavailable. Maintenance, a reboot, or a network issue. The server responds with code 421 or 451.
• Greylisting. The server deliberately rejects the first message from an unknown sender. Legitimate MTAs retry after a few minutes, and the second attempt goes through.
• Message too large. The attachment exceeds the server’s size limit. The server returns code 452 with a “Message too large” explanation.
• Rate limiting. The sender has exceeded the allowed sending rate. The server asks it to slow down.
• Security policy. The server requires TLS and the sender connects unencrypted. Or SPF/DKIM fails, but the DMARC policy is set to “quarantine” rather than “reject”.

SMTP codes for soft bounces

Codes starting with 4 indicate temporary failures:

• 421 — service temporarily unavailable, connection closing.
• 450 — action not taken, mailbox unavailable (temporary).
• 451 — processing error. Often accompanies greylisting.
• 452 — insufficient storage or limit exceeded.

Enhanced status codes: 4.2.2 means mailbox full, 4.7.1 means sending limit exceeded.

How retry delivery works

After a soft bounce, the sending MTA does not give up. It queues the message and retries on a schedule. A typical strategy: first retry after 15 minutes, then after an hour, then every 4 hours. The total retry window is usually 24 to 72 hours.

If no attempt succeeds within that window, the MTA generates a final bounce report (DSN) and stops trying. At that point, what started as a soft bounce effectively becomes a hard bounce.

Soft bounce vs. hard bounce

The primary signal is the SMTP response code: 4xx codes are soft, 5xx codes are hard. In practice it is not always clean. Some servers return 450 for a nonexistent mailbox, especially when greylisting is active. And code 552 (quota exceeded) gets classified as a soft bounce by some ESPs and a hard bounce by others.

A useful rule for email marketers: one soft bounce is normal. Three soft bounces in a row across different campaigns is a signal to remove the address. A mailbox that is consistently full belongs to someone who is not reading it.

Reducing your soft bounce rate

Some causes are outside your control: a recipient’s full mailbox, planned maintenance on their server. Others are not:

1. Configure SPF, DKIM, and DMARC. Missing authentication is a common reason servers reject mail on policy grounds.
2. Respect sending limits. Do not push 100,000 messages in an hour from a new IP. Warm up IP addresses gradually.
3. Remove addresses with repeated soft bounces. Three failures in the last 30 days is reason enough to suppress the address.
4. Watch message size. An HTML newsletter with heavy images can exceed the size cap on certain servers.

Soft bounce during email validation

During SMTP verification, a 4xx response usually means the result is inconclusive. The validator cannot confirm whether the mailbox exists. Those addresses get an “unknown” status. A good validator retries after a delay to rule out greylisting as the cause.