Why emails bounce (and how to handle bounces)

An email bounces when the receiving server cannot or will not deliver it, and sends back a delivery failure notice. Bounces matter because a high bounce rate tells mailbox providers you are sending to people you do not know, which can hurt your sender reputation.

Hard bounces and soft bounces

Delivery notices usually carry a status code. Codes starting with 5 (for example 5.1.1, "user unknown") are permanent. Codes starting with 4 are temporary.

How Distro detects bounces

Bounce notices come back to the mailbox that sent the email, usually from a "Mail Delivery Subsystem" or "mailer-daemon" sender. Distro reads these notices when it syncs your connected mailbox and classifies them:

• A notice with a permanent (5.x.x) status is treated as a hard bounce.
• A notice with a temporary (4.x.x) status, or with no readable code, is treated as a soft bounce. Distro treats unclear cases as soft on purpose, so it never blocks a good address by mistake.

What happens after a bounce

Hard bounce:

1. The prospect's active sequences stop, so no more follow-ups go to that address.
2. The address is added to your Unsubscribed list under People with the reason Bounced, and the contact is marked as bounced.
3. Future campaigns leave the address out automatically.
4. The bounce counts toward Bounced in the campaign and on Analytics.

Soft bounce: Distro records it, but does not block the address. If the same address keeps soft-bouncing, treat it as bad and remove or block it yourself.

For how bounces interact with the rest of a sequence, see What happens when a prospect replies, unsubscribes or bounces.

Why your emails bounce

• Guessed or outdated addresses. People change jobs; pattern-guessed addresses (firstname@company.com) are often wrong.
• Typos and formatting errors in your CSV.
• Old lists. The older the list, the more dead addresses it contains.
• The recipient's server blocks you. Some bounces mean "we don't accept mail from you", often because of missing authentication or a poor reputation. Check SPF, DKIM and DMARC.

How to reduce bounces

1. Use recent, sourced addresses. Avoid purchased lists.
2. Before importing, run the list through an email verification service of your choice. Distro checks format and flags role and disposable addresses as Risky, but it does not check whether a mailbox exists.
3. Start a new list with a small batch and check Bounced before sending to the rest.
4. Pause a campaign if bounces climb and clean the remaining contacts. See Pause, resume or end a campaign.
5. Never re-add people who bounced. If an address was wrong, find the correct one instead.