It’s not necessary for you to manually update or delete contacts who unsubscribe from your mailing list. When subscribers click on the 'Unsubscribe' link in your email, the system automatically updates your lists to keep a record of which contacts not to send to.
When a contact unsubscribes, they’ll still be part of your mailing list but won’t be sent any mail. This could be the reason why some contacts do not receive your mail.
You can manually unsubscribe contacts by clicking Contacts > Manage. Use the Search function to find the relevant email address and set the contact to 'Unsubscribed' so they won't receive any emails sent to that particular list. If you set the master status to 'Off', the contact won't receive any mail, regardless of the list they're in.
You can ensure that you're aware of unsubscribes by setting up notifications. Take a look at Lists Settings for more information. We also recommend taking a look at our article on Managing Lists.