Contact Lists
Posta does not provide a "contact list" resource. Contacts are auto-tracked recipients — they are created and updated by the system as you send mail, and are read-only. You cannot manually create lists of contacts or add/remove members.
If you are looking to group recipients, you almost certainly want one of the dedicated resources below.
Reusable mailing lists → Subscriber Lists
To build audiences you can target with campaigns and let people subscribe to or unsubscribe from, use Subscriber Lists. Subscribers are first-class records (with custom attributes and subscription state), and lists can be static or dynamic (segment-based).
See Subscriber Lists and Subscriber Management.
Opt-out groups → Unsubscribe Lists
To give recipients a granular way to opt out of one category of mail (for example "Product updates") without blocking everything, use Unsubscribe Lists. A send references an unsubscribe list so Posta can mint a one-click link that suppresses the recipient on that list only.
See Unsubscribe Lists.
Blocking individual addresses → Suppression List
To stop sending to a specific address (globally or for one unsubscribe list), use the Suppression List. Hard bounces and complaints add addresses here automatically.
There is no contact-lists API. Contacts are tracked automatically; grouping and opt-out are handled by Subscriber Lists and Unsubscribe Lists.