I would be interested to hear any advice or experience of including a checksum link in all emails sent to our various newsletter lists. We're interested in doing this because we have quite a variety of email lists on different topics. This means an opt-out link (which we obviously include as well) is quite a blunt tool for controlling what sort of content people receive. We would rather regularly offer them the option of updating their preferences if they're not getting the email newsletters they want from us. Does anyone include a regular email checksum link in their emails? Have you run into any problems doing so?