Email Bounce Management & List Hygiene

Bounce Types

Common SMTP Bounce Codes

550 5.1.1 — User unknown (hard bounce — remove address)
550 5.7.1 — Message rejected (spam filter or policy)
552 5.2.2 — Mailbox full (soft bounce — retry later)
421 4.7.0 — Temporarily rate limited (slow down sending)
450 4.2.1 — Mailbox disabled (soft → remove after retries)

Automated Bounce Processing

Your email service provider (ESP) handles most bounce processing automatically, but if you run your own mail server:

# Pseudocode for bounce handling
def process_bounce(bounce_event):
    if bounce_event.type == "hard":
        # Permanent failure — never send again
        subscriber.status = "hard_bounced"
        subscriber.suppress()

    elif bounce_event.type == "soft":
        subscriber.soft_bounce_count += 1
        if subscriber.soft_bounce_count >= 3:
            # Persistent soft bounces → treat as hard
            subscriber.status = "soft_bounced"
            subscriber.suppress()

    elif bounce_event.type == "complaint":
        # User marked as spam — critical
        subscriber.status = "complained"
        subscriber.suppress()  # Required by law

List Hygiene Practices

Regular Cleaning

• Remove hard bounces — Immediately, no exceptions.
• Suppress complaints — Anyone who marks you as spam is permanently removed.
• Re-engagement campaigns — After 90 days of no engagement, send a "do you still want to hear from us?" email. Remove non-responders.
• Sunset inactive subscribers — Remove addresses with no engagement after 6-12 months.

Prevention

• Double opt-in — Confirm email address before adding to list.
• Email validation at signup — Use syntax and MX record checks.
• Avoid role addresses — info@, admin@, support@ have high complaint rates.
• Honor unsubscribes instantly — Process within 24 hours (legally required within 10 days in CAN-SPAM).

Monitoring List Health

Tools for Verification

Before sending to an old or purchased list (which you should not purchase), verify addresses:

• ZeroBounce, NeverBounce, BriteVerify — Bulk email verification services.
• MX record check — Verify the recipient domain has active mail servers.
• SMTP check — Connect to the recipient's mail server and verify the address without sending.