Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
If you go to https://account.microsoft.com/services , can you sign in with your e-mail (@optonline.net) there? Do you see your subscription listed? This would usually work even if you can't actually send e-mails using that account, assuming that the Microsoft account which is using your e-mail as a user name is still intact.
If the subscription is there, turn off recurring billing, if it is currently enabled, and then you should be able to cancel the subscription.
If the account is inaccessible, you have to try and recover it. See Dave's reply.