What is a suppression list?
A suppression list holds the people and domains a campaign must not reach: people who unsubscribed or complained, current customers, leads already in another sequence, competitors. Checking each new lead list against it before sending is how an opt-out keeps working when the same person turns up in the next list you buy or build.
Reading the result
Load the leads, then one or more suppression lists, each named. An entry with an @ suppresses that address. A bare domain, @domain or a website address suppresses everyone at that domain and its subdomains, so acme.com also covers uk.acme.com. You get the clean list and the matches, each naming the lists and entries that caught it. Nothing is uploaded.
Common problems it finds
Opt-outs that come back
Someone who unsubscribed and then appears in a fresh export has still opted out. CAN-SPAM gives 10 business days to honour an opt-out, and under GDPR article 21 personal data is no longer used for direct marketing once its owner objects.
Customers in a prospect list
A cold pitch to a current customer tells them nobody checked. Suppress their domains as well as their contacts, so a colleague of the person you know is covered too.
A domain entry that catches too much
A domain removes everyone at it. A customer file with gmail.com in its website column would remove every Gmail lead. The entries that matched most are listed with their counts, so check them before you download.
The wrong column
Values that are neither an address nor a domain, such as names or phone numbers, are counted as ignored and shown, so a list read from the wrong column is obvious at once.