In case of dispute, it is up to the sender of the e-mail to prove the recipient's consent. With a double opt-in, this is not too much of an effort for you. If you use a provider, such as Mailchimp, for your newsletter marketing, the provider will record the following in the backend: E-mail address of the user, date and time of the opt-in, date and time of the double opt-in. This data can be used to prove the active consent of the user. If you do not use a provider service, it is recommended to document the data independently in a similar structure. It is also advantageous if this is done under the dual control principle, because a CSV file or an Excel spreadsheet can easily be modified and would therefore have little credibility in court. Ideally, your company has a certified data protection officer who is entrusted with this task and records the data after receiving the double opt-in confirmation.