Hard Bounce vs Soft Bounce: How to Protect Your Sender Score

Two Types of Email Bounces: Clear Definitions

When an email isn’t delivered, the recipient’s mail server sends back an error code. Depending on the nature of that error, you get either a hard bounce or a soft bounce.

Hard Bounce

A hard bounce means the email cannot be delivered permanently. The most common causes:

• The email address doesn’t exist (550 5.1.1 The email account does not exist)
• The domain doesn’t exist or has no MX server
• The destination server has permanently blocked your IP or domain

Hard bounces are the most dangerous. A single send to an invalid address is enough to trigger an alert with ISPs. An accumulation of hard bounces above 2% on a campaign is a red flag for Gmail, Outlook, and other providers.

Soft Bounce

A soft bounce is a temporary delivery problem:

• The mailbox is full (452 4.2.2 Mailbox full)
• The recipient’s server is temporarily unavailable
• The email is too large for the target server
• Temporary anti-spam rules are blocking your message

Soft bounces shouldn’t be ignored — an address that bounces softly across 3 consecutive campaigns should be treated as a potential hard bounce.

Sender Score: Understanding the Mechanics

The Sender Score (or “sender reputation”) is an index between 0 and 100 representing the reliability of your sending infrastructure according to ISPs and spam filters. It depends on several factors:

A high Sender Score (>90) means your emails land in the inbox. Below 70, you start seeing emails in spam. Below 50, some ISPs reject your emails outright.

Tools to measure your Sender Score:

• Validity Return Path (senderScore.org)
• Google Postmaster Tools (for Gmail)
• Microsoft SNDS (for Outlook/Hotmail)

How Bounces Degrade Your Reputation

The Cascade Effect

1. You send to 10,000 addresses — 300 of which are invalid (3% hard bounce)
2. Receiving servers record that you sent to invalid addresses
3. Your sending IP gets flagged as “unreliable” in ISP databases
4. Future emails from that IP increasingly end up in spam
5. Your open rates drop, further damaging your reputation
6. You end up on blacklists (Spamhaus, Barracuda, etc.)

This cascade can happen in 2 to 3 campaigns if your list isn’t properly qualified. Recovering a damaged reputation typically takes 3 to 6 months of active work.

Critical Thresholds to Monitor

Gmail tip: Since 2024, Google requires that the spam complaint rate stays below 0.1% to maintain good deliverability on Gmail (which accounts for ~40% of professional inboxes).

Protecting Your Reputation: 5 Essential Actions

1. Validate Addresses Before Adding to CRM

Prevention is the best defense. Integrate real-time email validation into your signup forms. The Syvel API instantly checks whether an address is valid, whether the domain has an MX record, and whether the address is known to be disposable.

curl https://api.syvel.io/v1/check/[email protected] \
  -H "Authorization: Bearer sv_your_key"

2. Immediately Delete Hard Bounces

After every campaign, export hard bounces and permanently delete them from your list. Don’t “pause” them — an invalid address will stay invalid.

-- Example: delete contacts with hard bounce
DELETE FROM contacts WHERE bounce_type = 'hard';

3. Process Soft Bounces After 3 Occurrences

A contact who soft bounces on 3 consecutive campaigns very likely has an abandoned mailbox. Quarantine them and attempt reactivation with a dedicated email.

4. Implement Double Opt-In

Double opt-in eliminates typos and fake addresses at signup. Yes, it reduces your conversion rate by around 20–30%, but it dramatically improves your list quality.

5. Clean Your List Every 6 Months

Addresses naturally become invalid: layoffs, abandoned accounts, domain changes. A valid address in January can be a hard bounce by July. Schedule regular cleanups.

The Role of Email Authentication

SPF, DKIM, and DMARC don’t directly reduce bounces, but they allow ISPs to trust your infrastructure and correctly attribute your reputation to your domain.

• SPF: lists servers authorized to send on behalf of your domain
• DKIM: cryptographic signature that ensures message integrity
• DMARC: policy for handling emails that fail SPF/DKIM

Without these three layers, even a clean list can suffer from deliverability issues related to domain spoofing.

Conclusion

Hard bounces and soft bounces are signals that your contact list needs attention. Ignoring them means letting your Sender Score progressively deteriorate until you can no longer reach your legitimate recipients.

The golden rule: prevent rather than cure. Entry-level validation (forms, imports, API) costs infinitely less than a reputation recovery campaign. To go further, learn how to clean your B2B email list before a cold email campaign and discover what catch-all addresses mean for your deliverability.