Should I ever send to a hard-bounced address again?

No. Hard bounces indicate permanent problems. Attempting to send again wastes resources and harms your reputation. Even if a subscriber contacts you claiming their address works, have them re-subscribe through your normal signup process with email verification.

What causes hard bounces?

Common causes include: typos in email addresses (gmial.com instead of gmail.com), fake addresses entered to access gated content, old addresses where the account was deleted, non-existent or expired domains, employees who left an organization, or addresses blocked at the server level.

What is Hard Bounce? - Definition & Guide

Definition

A hard bounce is a permanent email delivery failure that occurs when an email address is invalid, the domain does not exist, or the receiving server has permanently rejected the message. Unlike soft bounces, hard bounces will never succeed on retry and indicate the address should be removed from your list immediately. Hard bounces return SMTP error codes in the 5xx range.

Why It Matters

Hard bounces are the most damaging type of bounce for sender reputation. They indicate poor list quality and can suggest you are sending to purchased or outdated lists. Email providers closely monitor hard bounce rates as a spam signal. Even 1% hard bounces can trigger reputation warnings.

How It Works

When you send to an invalid address, the receiving server responds with a permanent failure code (5xx series). Common codes include 550 (mailbox not found), 551 (user not local), and 553 (mailbox name invalid). Your ESP should mark these as hard bounces, suppress the address, and prevent any future sends to that address.

Example

Common hard bounce scenarios:

1john.smithh@company.com - typo with double 'h', user meant john.smith@company.com. Returns "550 5.1.1 User unknown"
2sarah@oldstartup.com - the startup shut down and the domain expired. Returns "550 Domain not found"
3mike@bigcorp.com - Mike left the company 6 months ago and his mailbox was deleted. Returns "550 Mailbox does not exist"

All three are permanent failures that will never succeed.

Best Practices

1Remove hard bounces immediately, never attempt to send again
2Use double opt-in to prevent typos and fake addresses at signup
3Verify email addresses with an email validation service before importing
4Monitor your hard bounce rate closely, aim for under 0.5%
5Investigate sudden spikes in hard bounces (may indicate list issues)

The ranges and timings below are illustrative planning examples, not measured industry benchmarks or universal targets. Set thresholds using your own audience, product cadence and historical data.

Hard Bounce Code Table

SMTP code	Common meaning	List action
550	Mailbox unavailable, access denied, or policy rejection	Inspect the enhanced status code and text; suppress confirmed invalid recipients
551	User not local or forwarding unavailable	Suppress unless corrected by subscriber
553	Invalid mailbox name	Suppress and review signup validation
554	Transaction failed or rejected by policy	Inspect details before classifying

Hard Bounce Source Table

Source	Why hard bounces happen	Prevention
Signup forms	Typos or fake addresses	Double opt-in and real-time validation
Old CRM imports	Contacts changed jobs or domains expired	Verify before first send
Purchased lists	Invalid and non-consented addresses	Do not use purchased lists
Event lists	Manual entry errors	Confirm consent and clean before importing

Hard Bounce Threshold Table

Hard bounce rate	Meaning	Response
Below 0.5%	Strong list quality	Continue normal hygiene
0.5-1%	Watch closely	Review sources creating bounces
1-2%	Reputation risk	Clean list and tighten acquisition validation
Above 2%	Serious list quality problem	Pause risky sends and suppress bad sources