Soft Bounce
A soft bounce is a temporary email delivery failure: the receiving server accepted the connection but declined the message for a transient reason (a full mailbox, a server that is briefly down, or rate-limiting) and signalled it with a 4xx SMTP reply. Unlike a hard bounce, it isn’t final; your mail server queues the message, retries it, and often delivers it on a later attempt.
- Temporary, so the message can still arrive on a later retry
- Your mail server retries automatically; no manual resending needed
- A rising soft-bounce rate can be an early reputation warning
- Suppress any address that soft-bounces on every send
4xx (e.g. 421, 450, 452)
What a soft bounce actually is
Every SMTP transaction ends with a three-digit reply code. Codes in the 5xx range mean a permanent failure, or hard bounce. Codes in the 4xx range mean a transient failure, or soft bounce: the server is saying “not right now, try again later,” not “never.”
When your mail transfer agent receives a 4xx response it doesn’t discard the message. Instead it holds the message in a queue and retries on a back-off schedule, typically for 24 to 72 hours (the exact limit is configurable; Postfix, for example, defaults to five days). If the message still hasn’t been accepted when the queue lifetime expires, it is finally returned to the sender as a hard bounce.
Common causes, and what each 4xx code means
A soft bounce usually arrives with a reply code that hints at why it failed:
421: service not available, or the server is closing the connection. Often a sign of throttling.450: mailbox unavailable or busy (commonly a temporary lock or greylisting).451: local error in processing on the receiving side.452: insufficient system storage, or too many recipients in one transaction.
The underlying reasons cluster into a few buckets: the recipient’s mailbox is full, the receiving server is temporarily down or overloaded, the message is too large, the sender is being greylisted, or, importantly, the receiver is deliberately rate-limiting you because of volume or a reputation concern.
Soft bounces and your sender reputation
A handful of soft bounces is completely normal; mailboxes fill up and servers reboot. The danger is treating every 4xx as harmless noise, because some are reputation signals in disguise. When Gmail or Yahoo return 421 4.7.0 … try again later, they are often throttling you on purpose: you have sent too fast, or your domain/IP reputation has slipped. Ignoring a rising soft-bounce rate means missing an early warning before mail starts landing in spam.
Persistent soft bounces also quietly erode list quality. An address that soft-bounces “mailbox full” on every send for weeks is effectively dead, and many recycled spam traps begin life as abandoned mailboxes that soft-bounce before they are repurposed. Continuing to hammer those addresses tells mailbox providers you aren’t practising list hygiene.
How to handle soft bounces
- Let the retries happen, but cap them. Don’t manually resend; your MTA already retries. Do set a sane maximum so a message isn’t retried for days.
- Convert repeat offenders. If an address soft-bounces across several consecutive campaigns (a common rule is 3 to 5 sends), treat it as a hard bounce and move it to your suppression list.
- Watch the reason codes, not just the rate. A spike in
421throttling responses from one provider is a reputation problem to investigate, not a list problem.
How a soft bounce plays out
4xx replySoft bounce vs hard bounce
| Soft bounce | Hard bounce | |
|---|---|---|
| Duration | Temporary | Permanent |
| What to do | Let it retry | Stop sending |
| Typical cause | Mailbox full, throttling | Address does not exist |
| SMTP code | 4xx |
5xx |
| Recoverable? | Often | No |
| Reputation impact | Low if occasional | High, remove at once |