What a suppression list is
A suppression list is a list of people who must not be sold or contacted. Every new lead is checked against it before it goes anywhere. If a lead matches an entry, it is suppressed: kept back from one buyer, or from every buyer, depending on where the list applies.
Suppression is close to deduplication, but it answers a different question. Dedupe asks whether you have already sold this person. Suppression asks whether you are allowed to sell them at all. A dedupe window is a commercial choice. A suppression list usually comes from a customer, a complaint or someone asking not to be contacted.
What goes on a suppression list
Most operators end up holding several lists, each with a different owner and a different reason for existing.
Opt-outs
People who have asked not to be contacted. Keeping them on a list means a request made to one campaign is still respected in the next one.
A buyer's existing customers
A buyer does not want to pay for people who already buy from them. Many will send you a customer file, often hashed, to screen against.
Complaints and disputes
Anyone who has complained about contact, or whose data is in question, should stop flowing to buyers while it is dealt with.
Known bad data
Test entries, staff details, fraudulent submissions and numbers buyers have rejected again and again.
Not every list is one you build. In the UK, the Telephone Preference Service (TPS) and the Corporate Telephone Preference Service (CTPS) record people and organisations who do not want unsolicited sales calls. Screening against those registers is a separate check from your own suppression lists.
Hashed suppression lists
Buyers are often unwilling to send their customer list to a broker in plain text, and with good reason. The usual answer is a hashed list. Instead of the email address itself, the buyer sends the output of a hash function, most often MD5 and sometimes SHA-256. To check a lead, you hash its email or phone number in the same way and compare the two results.
Two things are worth knowing. First, a hash is not encryption. There are at most around a billion UK mobile numbers, so a hashed phone list can be reversed by hashing every possible number and looking them up. Emails are much harder to reverse because they cannot be listed in the same way. Treat hashing as a way for two parties to compare lists, not as a way to keep one secret.
Second, a hash only matches if both sides hashed exactly the same text. [email protected] and [email protected] produce completely different digests, and so do 07700900123 and +447700900123. Agree the format with whoever sends the list: usually emails trimmed and lower-cased first, and phone numbers in a stated format. A hashed list that matches nothing looks exactly like a list nobody is on, so test it with a few entries you know are on it.
How matching works
Most suppression matching is on email address, phone number or both. Matching on either one catches more people: someone who changed their email but kept their phone number is still caught.
Normalising matters here as much as it does for dedupe. Emails should be compared regardless of capital letters and stray spaces. UK phone numbers turn up as 07..., 447... and +447... depending on the system that exported them, so compare them in one agreed format, or in all of the common ones.
Some lists use other fields: a postcode, a property reference such as a UPRN, or a company number. Those work the same way, as long as the field on the lead is filled in consistently.
Where to check
At intake, when the list applies to everyone. A person who has opted out of all contact should be turned away when the supplier posts the lead, so the supplier knows it will not be paid for.
Per buyer, when the list belongs to one buyer. A buyer's customer list only matters for that buyer. The same lead may be a perfectly good sale for someone else, so the check should skip that one buyer, not the whole lead.
Before a bulk send. If you are sending a file of data to a client, run it against their list first.
Keeping lists current
Suppression lists go out of date. Agree with each buyer how often they send a new file, and replace the old one when it arrives. Record where each list came from and when, so you can answer questions about it later. Opt-outs you receive yourself should go onto your list straight away.
If you are unsure what your obligations are for a particular list or register, take advice from a compliance specialist. This page explains how suppression works, not what the law requires of your business.
How it works in Hawfinch
In Hawfinch, any campaign can be used as a suppression list. You load the list into a campaign, usually by importing a CSV, and then choose which of those campaigns each buyer is suppressed against. Lists you upload for matching do not count towards your lead allowance.
Suppression is set on each buyer in a delivery group. Before routing starts, Hawfinch checks the lead against every list its buyers use, matching on email and phone. A buyer whose list contains the lead is skipped, and the lead carries on down the waterfall to the next buyer, so one buyer's customer list never costs you the sale to another. For a list that should apply to everyone, such as your own opt-outs, attach it to every buyer in the campaign.
Each list can be plain, MD5 hashed or SHA-256 hashed. You mark the format on the campaign that holds the list, and Hawfinch hashes each incoming lead the same way before comparing. Emails are trimmed and lower-cased first, and phone numbers are checked in the common UK formats (07..., 447..., +447... and without the leading zero), so a list exported from another system still matches. Keep one campaign per format: a file hashed a different way needs its own.
To build a hashed list yourself, open the file in Files and hash any column to MD5 or SHA-256. Files can also suppress one file against another before you send anything out.
Suppression fails closed: if a check cannot run, the lead is held back rather than sent unchecked. When you do need to resend a lead regardless, you can redistribute it with suppression skipped for that one send. To block on other fields, such as postcodes or UPRNs, use a reference list filter on a group or buyer. TPS and CTPS screening is coming soon. There is more on the lead management page, and the full list of checks is on features. Every one of them is on every plan, as shown on pricing.