Deliverability
Updated
Soft bounce
Definition
A soft bounce is a temporary delivery failure: the receiving server deferred the email but the address may accept it later. The sending server retries automatically before giving up.
How it works
Temporary failures use SMTP reply codes in the 4xx range. Common causes are a full mailbox, a server that is temporarily unavailable, rate limiting because you sent too much too fast, and greylisting, where a server deliberately defers the first attempt from an unknown sender.
The sending server keeps the message in its queue and retries with increasing delays. RFC 5321 suggests giving up after at least 4 to 5 days. If it still fails, the message becomes a final bounce.
Some providers return a permanent 5xx for a full mailbox (5.2.2), so the line between soft and hard depends on the receiver.
Example
4xx codes mean retry later. 4.7.0 often signals rate limiting.
Why it matters
A few soft bounces are normal. A spike, especially rate-limit deferrals from one provider, means that provider is slowing you down, usually because of volume or reputation.
An address that soft bounces on every send for weeks is effectively dead and should be handled like a hard bounce.
Best practices
- Let your ESP retry. Do not resend manually, which only adds load.
- Watch deferrals per mailbox provider. A cluster at one provider points to throttling, not bad addresses.
- Suppress addresses that keep soft bouncing across several consecutive sends.
- Spread large sends over time on new domains and IPs to avoid rate limits.
How Lumail handles soft bounce
Temporary failures are retried by Lumail's sending infrastructure on Amazon SES. Recipient rate guards add a 24-hour cooldown after a mailbox-full or recipient-throttled bounce, so one struggling inbox is not hammered.
Delivery delays caused by deferrals have their own alert and fix guide in Lumail's deliverability docs.
Check a real message with the free mail tester or the spam tester.
Frequently asked questions
Do soft bounces hurt sender reputation?
Occasional ones do not. Persistent rate-limit deferrals are a symptom of reputation or volume problems, and continuing to push volume into them makes things worse.
How many soft bounces before I remove a subscriber?
There is no universal number. A common rule is to suppress an address after it soft bounces on three to five consecutive sends.
What is greylisting?
An anti-spam technique where a server temporarily rejects the first message from an unknown sender with a 4xx code. Legitimate servers retry and get through. Spam tools often do not.
Related terms, tools and docs
- DeliverabilityHard bounceA hard bounce is a permanent delivery failure: the receiving server rejected the email and retrying will not help.
- MetricsBounce rateEmail bounce rate is the percentage of sent emails that the receiving server rejected as undeliverable.
- DeliverabilityEmail warmupEmail warmup is the practice of gradually increasing sending volume from a new domain or IP address so mailbox providers can build a positive reputation for it before you send at full volume..
- DeliverabilitySender reputationSender reputation is the trust score mailbox providers assign to your sending domain and IP addresses, based on how recipients and their systems react to your mail.
- Free toolFree mail testerSend a real email and get an SPF, DKIM, DMARC and content report.
- DocsHow to fix delivery delaysDeferrals, throttling and retries.
- DocsRecipient rate guardsPer-address caps and bounce cooldowns.
- DocsHow to lower bounce rateWhat to do when permanent bounces pass 2% or 5%.
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