You can also update the profiles of existing contacts using an online form.
HeyForm offers you a special type of form for this: the profile update form which you can use to collect information about your subscribers, such as contact details or preferences and interests, even after the fact.
As a general rule, the more personalized your newsletter campaign, the more successful your performance.
Therefore, find out as early as possible what your contacts are really interested in. The easiest way to do this is to use checkboxes and create multiple lists (keyword: segmentation).
What exactly can you query? Let your contacts choose between different mailing frequencies (e.g. weekly, monthly, or only during sales). Or ask your customers which products they would like to receive regular updates on.