The Digital Marketing Glossary
An "opt out" is another phrase for an "unsubscribe" — a request to be removed from an email list.
An opt-out request may apply to a specific set of messages (e.g. a drip campaign for a specific upcoming event), or it may be a blanket opt-out request to stop receiving any and all email from the sender.
To comply with the CAN-SPAM Act, an opt out link needs to be included in every email marketing campaign. Opt out requests need to be honoured within 10 days of the sender receiving them.
The vast majority of modern email marketing platforms offer a way for recipients to opt out of emails. The best email marketing platforms make it easy for subscribers to opt out — it's better for everyone if a subscriber can opt out easily, otherwise they will turn to the "mark as spam" button — which can lead to reputation and deliverability issues down the line.