Hi @TWeeks,
Generally, there’s this catch 22 of an opt-in email requiring a legal basis that you are just abou to obtain. Technically, if there is no legal basis to send such an email, you cannot ask contacts to provide such a legal basis.
Whether there is a legitimate interest is something that should always be judged by a legal professional (legal team, data privacy officer, external legal consultant). Typically, yes, an existing client relationship can constitute a legitimate interest. Whether a confirmation is needed on top of that depends on the legislation you’re subject to.
And yes, the email can be customized and sent via workflow. For this, you would create a regular automated email. When you add a link or button in that email, you can change that link to a double opt-in link:
If I recall correctly, emails with such a link should always be sent as an automated email, never as a regular email.
Best regards!