How to handle soft bounces vs hard bounces correctly

Bounces are not all the same. The mail provider returns a specific status code that tells you whether the failure is permanent (hard) or temporary (soft). Treating them differently is essential. Handling soft bounces like hard bounces costs you legitimate recipients. Handling hard bounces like soft bounces wrecks your bounce rate.

Hard bounces (5xx codes)

Permanent delivery failures. The address is gone and will not come back.

• 550: mailbox does not exist
• 551: user not local
• 553: mailbox name not allowed
• 554: transaction failed (varies by server)

Action: remove from your list immediately. Continuing to send to confirmed hard-bounced addresses is the fastest way to destroy your sender reputation. Every reputable ESP automatically suppresses hard-bounced addresses after one bounce. If your ESP does not, configure it or switch.

Soft bounces (4xx codes)

Temporary failures. The address may work later.

• 421: service not available, try later
• 450: mailbox unavailable (e.g. greylisting)
• 451: local error in processing
• 452: insufficient storage (inbox full)

Action: retry on a schedule. Standard practice:

• First soft bounce: retry in a few hours.
• Second consecutive soft bounce: wait 24 hours, retry.
• Third consecutive soft bounce: suppress (treat as hard).

Every major ESP handles this automatically. Configure the suppression threshold if your ESP exposes it.

How verification helps

Pre-send verification catches the vast majority of hard bounces before you ever send. They show up as invalid or disabled and you drop them from your list before the campaign. The hard bounces remaining are addresses that went bad between verification and send (small percentage), or edge cases the engine could not catch.

Soft bounces are harder to predict pre-send because they are state-dependent (inbox-full status, temporary server outages). Verification flags some, since inbox_full results indicate the mailbox was over quota at verification time, but most soft bounces happen during the send itself.

Tracking bounce-rate metrics correctly

When ESPs report your "bounce rate", they typically include both hard and soft bounces. Some platforms separate the two. If yours does, watch them independently:

• Hard bounce rate above 1% means a list quality problem. Verify.
• Soft bounce rate above 2% means sending infrastructure or recipient-server issues. Check authentication, throttling, and the time-of-day of your sends.