The unsubscribe rate shows what share of recipients unsubscribed after a message, and the complaint rate shows how many marked the message as spam. The difference between them is fundamental. Unsubscribing is a normal, healthy process that cleans the database and does not harm deliverability. A complaint is a direct signal to mail providers that you are sending unsolicited communication, and it takes very few of them to damage the domain's reputation. People complain mainly when they cannot find the unsubscribe link, when they do not remember signing up, or when the frequency is unbearable. Prevention is therefore simple: a visible unsubscribe link in every message, a reminder of the contact's origin, a reasonable frequency, and the option to choose what the recipient wants to receive messages about.
See also: Email Deliverability, Contact list hygiene, Legal basis for email marketing.
