How a list works

A list is the unit of subscription. Everything you send — a campaign, a sequence — targets one list, and everything a contact decides applies inside that list. The rule that follows from it: a contact belongs to exactly one list. The same address in two lists is two separate contacts, with two histories and two unsubscribe links.

How you use one

Create as many lists as you have distinct audiences — a newsletter, a customer base, a waiting list. What makes a second list right is that the two audiences would never receive the same email, and that leaving one should not mean leaving the other. Inside a list, an email address is unique. Adding an address that is already there updates the existing contact instead of creating a second one — through the interface, through an import, or through the API. The rest of the audience vocabulary works differently:
  • Tags cross lists and describe the person.
  • Segments live inside a single list and select part of it.
  • Topics belong to the organization and let a reader decline one theme without leaving the list.

What it means for unsubscribing

An unsubscribe stops emails for that list. If the same person is also a contact in another list, that other subscription is untouched and they keep receiving from it. That is a deliberate choice, not an oversight: two lists were two consents. But it means splitting one audience into several lists multiplies the exits a reader has to find. Split when the audiences are genuinely different, not to organise your own filing.
Deleting a list deletes its contacts. Every contact of the list goes with it, along with their history and their choices — including the people who had unsubscribed, who then have nothing left protecting them if you re-import the same file. There is no undo.

What limits you

Two ceilings come from your plan: the number of lists an organization may create, and the number of contacts, all lists combined. Reaching the ceiling does not break anything already there: adding new contacts is refused, existing ones keep receiving. Import is refused the same way, before a single row is written. The count includes unsubscribed contacts, because their record is what keeps you from writing to them again. Deleting them to free room would let a later import re-create them as subscribed.

What it implies elsewhere

Because a campaign targets a list, the list determines the maximum audience of any send. A tag or a segment can only narrow it; nothing widens it. Sending the same email to two lists is two campaigns, with two sets of statistics.