Would something like a verification email asking them to double verify answer that? They click the box, then they have to open an email and click a link also verifying it?
But is that sufficient under GDPR? Although a double opt-in has generally been considered good practice for a long time, it only demonstrates that a recipient has agreed to receive mail for some purpose, not for any specific purpose.
Even if you've been building up your mailing list for years, following generally accepted good practices, and only signing up genuinely interested recipients, it seems you could now to be in a position where either:
(a) when you signed people up, you provided sufficient information about what you would be sending to them and you can still produce evidence of that today;
(b) you need to contact everyone on your list to obtain explicit, specific consent for whatever you actually send to your list; or
(c) you have to remove anyone who isn't covered by (a) or (b) above (or delete your whole list).
As with so much about the GDPR, what will be accepted as reasonable evidence of informed consent for earlier subscribers to a mailing list is ambiguous, and the consequences of either doing too much or not doing enough are undesirable.
Even if you've been building up your mailing list for years, following generally accepted good practices, and only signing up genuinely interested recipients, it seems you could now to be in a position where either:
(a) when you signed people up, you provided sufficient information about what you would be sending to them and you can still produce evidence of that today;
(b) you need to contact everyone on your list to obtain explicit, specific consent for whatever you actually send to your list; or
(c) you have to remove anyone who isn't covered by (a) or (b) above (or delete your whole list).
As with so much about the GDPR, what will be accepted as reasonable evidence of informed consent for earlier subscribers to a mailing list is ambiguous, and the consequences of either doing too much or not doing enough are undesirable.