SEmail Deliverability Glossary

Suppression List

A do-not-contact database of unsubscribers, complainers, and hard bounces. Sending to suppressed addresses during warm-up immediately damages reputation.

A suppression list is a do-not-contact database that ensures email addresses who have unsubscribed, complained, or bounced are permanently excluded from future sends.

What belongs on a suppression list:

All unsubscribers
Anyone who filed a spam complaint
All hard-bounced addresses
Spam trap addresses (if known)
Addresses that have explicitly requested removal

Critical for warm-up: Sending to suppressed addresses — especially complaint filers or known invalid addresses — during warm-up will immediately damage the reputation you're building. Suppression lists must be applied to both warm-up sends and campaign sends.

Frequently Asked Questions

How do I build and maintain a suppression list?

Your suppression list is built from four sources: unsubscribe requests (your ESP should capture these automatically when recipients click your unsubscribe link), FBL complaint notifications (add anyone who reports your email as spam immediately), hard bounce notifications (your ESP should automatically process these, but verify), and any manual removal requests received by other channels. Store your suppression list in a centralized location accessible to all your sending platforms. If you send from multiple ESPs or tools, synchronize the suppression list across all of them — a contact suppressed on one platform must be suppressed on all platforms.

What happens if I send to a suppressed address?

Sending to a previously complaining address is the fastest way to generate repeat complaints and destroy your reputation. Recipients who already marked you as spam once are extremely likely to do so again if they receive another email — and some spam filters give extra weight to repeat complaints from the same recipient. Sending to suppressed hard-bounce addresses will generate bounce events again, compounding your bounce rate metrics. Many ESPs monitor for suppression list violations and may suspend accounts that repeatedly send to suppressed addresses. Treat your suppression list as a permanent, non-negotiable do-not-contact database.

Should suppression lists be applied to warm-up sends?

Yes — but in practice, warm-up sends only go to your warm-up network's verified accounts, which should never include any of your real recipients. The relevant scenario is when you begin blending real prospect sends with warm-up — at that point, your suppression list must be applied to all real sends alongside your warm-up traffic. If you're using a warm-up tool that sends on your behalf, ensure the tool is configured to exclude suppressed addresses from any sends it makes using your account. A suppression violation during warm-up has the same reputation consequences as one during a full campaign.

Related Terms

Bounce Rate

The percentage of sent emails that could not be delivered and were returned to the sender. High bounce rates during warm-up immediately stall reputation building.

Complaint Rate (Spam Complaint Rate)

The percentage of recipients who click 'Mark as Spam.' Google's 2024 guidelines treat anything above 0.10% as a serious deliverability risk.

Feedback Loop (FBL)

A service offered by major ISPs that notifies senders when recipients mark their email as spam, enabling immediate list suppression.

Return Path (Bounce Address / Envelope From)

The SMTP envelope address where delivery failure notifications are sent. SPF checks the Return-Path domain — mismatches cause DMARC alignment failures.

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