Launching a soft opt in with old mailing list

[rinsethenerd-dotcom]

Listmonk version: v[xx] (install: Docker/binary)
OS/Infra: [e.g., Ubuntu 22.04 on EC2]
PostgreSQL: [version]
SMTP provider: [SES/Mailgun/etc]

Goal:Take an old mailing list, as the client for a company who has never launched a newsletter or done any email campaigns, from customers they've built on paper and over time through legit means.

What happened: Unable to send emails to customers without them being subscribed through listmonk.
Ideally I'd like to give their customers a chance to subscribe to the newsletter for offers, and remove all who wish to unsubscribe with a single soft opt in campaign.

What I've tried: sent a test campaign to a small number of emails which all came back as unconfirmed.

Tried to subscribe through listmonk with multiple emails and have never been sent an email to subscribe.