Lead lists
Lists keep people outside any campaign so the same set can start more than one. And every way a lead arrives runs through the same rules before the first send.

Upload a CSV to a named list. A list is a set of people, not a campaign, so it can start this campaign today and another one next quarter.
CSV, a published Google Sheet, API, manual entry or a list: each address goes through the same checks, in the same order. Not an address, already in this campaign, on your blocklist, already a lead in another campaign, contacted too recently.
On by default. An address that is already a lead in another campaign in the workspace is skipped and counted, so two uploads made before the first send cannot both go out to the same person.
Nobody is contacted again inside this many days, whatever campaign it is. Default 180, counted from the last email you sent them. Set it to 0 to turn it off.
Every upload reports what happened to each row: added, duplicates, invalid, blocked, in another campaign, contacted recently. A skipped row is visible. A second cold email is not recoverable.
Pause a single lead and resume it later. A lead you have already emailed cannot be deleted, only paused, so deleting a row can never free up a cap or restart a sequence.
One click checks every pending lead’s domain for a mail server and marks the dead ones invalid before anything is sent. It costs nothing and removes the guaranteed bounces from a stale list.
Download a campaign’s leads as CSV with status, step and dates, or search every lead in the workspace by address.
Narrow a campaign’s leads by status, interest, address check, lead list or a search on the address, and save the filter under a name. Saved filters belong to the workspace, so the same one works on every campaign’s leads with one click.
Addresses and whole domains, typed in or imported from a CSV. Unsubscribes land there on their own, and so does an address the receiving server says does not exist. Checked on every import and again at the moment of sending.