Hey @Juan Cruz Martinez77! It sounds like you are referring to unconfirmed subscribers (subscribers who signed up but never confirmed their subscription) rather than unsubscribers (subscribers who clicked the unsubscribe link in one of your emails to be removed from your list).
We actually have a guide on unconfirmed subscribers here: Why do I have unconfirmed Subscribers?75
It's normal to have some unconfirmed subscribers, and this actually helps protect your list! It's possible that some if not many are bots or invalid addresses which could affect your deliverability in the long run by lowering your overall engagement. We have more information on how double opt-in protects your list here: The All Important Double Opt-In68
That all being said, I'd also recommend double-checking that your account's sending address60 is an @customdomain email address. More info on that here68. Our deliverability lead has put together some great guides on deliverability best practices — I'll include those links below: