Updated · 8 min read
Bounces vs blocks vs deferrals: what your ESP's error codes actually mean
Your ESP reports a 2% bounce rate. Is that healthy, worrying, or a crisis? Depends entirely on what kind of bounces they are. Soft bounces from full mailboxes are fine; hard bounces from bad addresses are a list hygiene issue; blocks from reputation failure are a crisis. Most dashboards don't distinguish, and most programs act on bounces as if they were one thing. Here's the real taxonomy.
Justin Williames
Founder, Orbit · 10+ years in lifecycle marketing
The SMTP response code taxonomy
When your mail server tries to deliver to a recipient server, the recipient server responds with a status code. The first digit tells you the category:
2xx — Success. Mail accepted. Moves on.
4xx — Transient failure (deferral). Recipient server couldn't accept the mail right now but might later. Your server will retry automatically, usually for 24–72 hours. No action needed from you.
5xx — Permanent failure (bounce or block). Mail rejected and should not be retried. The subtypes matter: 5.1.x is usually "bad address" (hard bounce); 5.7.x is usually "policy / reputation" (block).
ESPs categorise these further into human-readable categories. Terminology varies by ESP, but the underlying SMTP codes are standard.
Hard bounces
Hard bounce: a permanent delivery failure tied to the address itself. The address doesn't exist (typo, deleted account, wrong domain), the domain doesn't exist, or the address has been disabled by the provider.
SMTP indicators: 550 "No such user", 550 "Recipient address rejected: User unknown", 553 "Bad destination mailbox", various domain-not-found responses.
What to do: suppress permanently. See rule 1 of the list hygiene policy. One hard bounce means the address is permanently bad; sending again produces another bounce and damages reputation.
Hard bounce rate should be < 2% on broadcast sends. A program with 5%+ hard bounce rate has list hygiene debt that damages deliverability every send.
Soft bounces
Soft bounce: temporary delivery failure. Mailbox full, recipient server temporarily unavailable, temporary authentication issue, message too large.
SMTP indicators: 4xx codes (deferrals) that eventually time out, specific 5xx codes like 552 "Requested mail action aborted: exceeded storage allocation".
What to do: retry on next send. Continue to attempt delivery; most soft bounces resolve within a send or two. Rule 2 of the hygiene policy is the handling: three consecutive soft bounces flags for suppression, because persistent soft bounces usually indicate an abandoned mailbox.
Blocks
Block: the recipient server rejected the mail not because the address is bad, but because of something about the sender (you). Bad IP reputation, content flagged as spam, authentication failure, SBL (spam blocklist) inclusion.
SMTP indicators: 550 5.7.1 codes with various messages, 550 "Rejected due to spam content", 550 "Sender reputation not acceptable", 554 "Transaction failed" with specific cause.
What to do: investigate urgently. A block means the receiving server actively refused your mail. If a small number of blocks from one ISP — may be temporary. If blocks from multiple major ISPs or climbing volume — you have a reputation or authentication problem.
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Deferrals
Deferral: 4xx response from the recipient server asking your server to try again later. Common causes: temporary server overload, rate limiting, greylisting (anti-spam measure that asks unknown senders to retry before accepting).
What to do: nothing. Your SMTP server retries automatically on a schedule (typically every 15 minutes for the first hour, exponentially longer after). Mail usually gets through within a few hours. No manual intervention.
The one exception: if deferral rate spikes to 20%+ and persists, there may be a rate-limiting issue — your sending volume to a specific ISP exceeded their acceptance rate. Your ESP's delivery team can help investigate.
The metrics to monitor
Hard bounce rate: < 2%. Above 5% on broadcasts indicates list hygiene problem. Driven by list quality — acquisition source, list age, dormant user inclusion.
Soft bounce rate: 2–5% typical on broadcasts. Rising sustained rate may indicate engagement problems (dormant users whose mailboxes are full or neglected).
Block rate: < 0.5%. Above 1% is a deliverability emergency. Driven by reputation, authentication, content.
Deferral rate: usually not worth dashboarding. Eventually resolves; monitor only if spiking sustainedly above 20%.
Most ESP dashboards show a single "bounce rate" aggregated across all these. Worth digging into the breakdown when troubleshooting; different root causes require different interventions.
Root-cause framework
When bounces rise:
1. Check which type is rising (hard, soft, block, deferral).
2. Hard: list hygiene issue → audit recent acquisition, audit dormant users, check import processes.
3. Soft: engagement or infrastructure → check whether dormant users are over-represented, check whether specific ISP is having issues.
4. Block: reputation or authentication → check Postmaster Tools, check IP/domain reputation, verify SPF/DKIM/DMARC, check content for spam triggers, check IP blocklists.
5. Deferral: usually transient, monitor.
Acting on "bounce rate" as a single number mixes these signals. Acting on the correct subtype produces faster, more accurate fixes.
The Deliverability Management skillcovers the incident-response playbook for each bounce type — treating each with the specific remediation rather than generic "improve deliverability" advice.
Frequently asked questions
- What's a healthy bounce rate?
- Hard bounce: under 2% of sends. Soft bounce: 2–5%. Block: under 0.5%. Aggregate bounce rate (what most dashboards show): under 5% for broadcast programs, under 2% for engaged-user-only sends.
- Should I suppress soft bouncers immediately?
- No. Soft bounces are often transient (full mailbox clears, temporary server issue). Continue sending; suppress after three consecutive soft bounces (the hygiene rule). Single soft bounce is not a suppression trigger; persistent soft bounce is.
- What do I do when block rate spikes?
- Emergency protocol. Stop broadcast sending; send only to users who engaged in the last 30 days. Investigate: Postmaster Tools for reputation signals, authentication records (SPF/DKIM/DMARC), IP blocklists (MXToolbox, Talos), recent content for spam triggers. Root-cause fix, then gradually resume volume. Recovery 2–6 weeks typical.
- How do I tell a soft bounce from a deferral?
- A deferral is a 4xx response with automatic retry; a soft bounce is when the retries eventually fail with a 4xx or the message ages out. Your ESP usually makes this distinction in reporting — 'deferred then delivered' vs 'deferred then bounced' are different outcomes.
- Why is my hard bounce rate high after a marketing push to old addresses?
- Bad list hygiene. Old addresses decay over time — users delete accounts, change ISPs, jobs end. Sending to a long-unused list shows high hard bounce rate immediately. Fix: segment by last engagement; only send to users with engagement in the last 12 months; sunset or remove older unengaged addresses.
- Do bounces affect sender reputation?
- Yes. Hard bounces signal bad list hygiene — ISPs use high hard bounce rate as a spam signal. Soft bounces less so, but still marginally. Blocks are the strongest negative signal because they're the ISP directly rejecting you. Reducing bounces is one of the most direct ways to protect reputation over time.
This guide is backed by an Orbit skill
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