Hi there, As one of the largest Constant Contact Certified Solution Providers we encounter these types of issues with clients frequently - you're not the only one! Consider NOT deleting contacts, but rather: Keep your contacts who have an email address in the account. This will ensure the system knows which email addresses unsubscribed. That's very important. Create a new spreadsheet where you include email addresses of new or current contacts. For each email address, include any new or updated information.  Upload the new spreadsheet and the system will sync all the data using the email address as the key. Remove any contact from the account that does not have an email address as those are useless for email marketing purposes.  You may also consider verifying your email list if it has not been used in over 6 months or there are new email addresses. This is something we strongly advise clients to do as it ensures that invalid email addresses are removed from the account, thereby reducing your bounce rate and increasing your deliverability. If you'd like to chat about any of this, please reach out. 888-840-2595 x701 ... View more