The suppression list

The suppression list holds the addresses that are excluded from every send, in every list. An address lands there for one of two reasons, and both come from the outside world:
  • hard bounce — the address does not exist, permanently;
  • spam complaint — the recipient pressed the spam button in their mail client.

Why you cannot take an address off it

The list is fed automatically by the sending provider, and it is read-only on purpose. Both events are counted against your sender reputation by the mail providers. Past a threshold, they stop delivering your mail — not to that address, to everybody. Sending again to an address that already hard-bounced is the fastest way to get there, which is why no button exists to try.
A complaint is worse than an unsubscribe, and it means the same thing. Someone who cannot find your unsubscribe link presses spam instead. A visible, honest unsubscribe link is what keeps your complaint rate low — and it is the one thing that protects delivery for every other reader.

How it relates to a contact

A suppressed address usually also has a contact in bounced or complained state. The two are not the same thing:
  • the contact state belongs to one list and can be seen on the contact;
  • the suppression belongs to the organization and outlives the contact — delete the contact, re-import the address, and it is still suppressed.
That is a feature. It is the only thing standing between a careless import and a reputation incident.

What it changes at send time

Suppressed addresses are removed when the send resolves its audience, not when you pick it. So the number of emails actually sent can be lower than the audience you saw — the difference is suppressions, unsubscribes and topic refusals. They never count as recipients, and they consume nothing.

What it implies elsewhere

A long suppression list is a signal about your source of contacts, not about your writing. Addresses that hard-bounce in bulk usually came from a purchased list, a scraped list, or an export that is several years old. Nothing in the product lets you clear the list, and nothing should. If you believe an address is there by mistake, the answer is a fresh, genuine sign-up from its owner.