If you want a single opt-in autoresponder your best bet is your own autoresponder script like this one.

A single opt-in means someone sees your newsletter signup form, enters their email, and is automatically approved to receive messages.

There are obvious risks involved in this, which is why major online autoresponder services discourage and not allow the practice. If they allow it and receive a complaint, you could be looking at a quick ban.

- Typos. I...