When to Add an Unsubscribe Link

Under CAN-SPAM, GDPR, and sender policies enforced by Google and Yahoo:

Marketing & Promotional Emails: Must include a clear, working unsubscribe link in the footer and RFC 8058 one-click headers. Mailofly automatically includes these on all Campaigns.
Transactional Emails: Password resets, account activation, receipts, and security alerts do not require an unsubscribe link, as recipients need these messages to use the product.