@UnaBomba
Welcome to the Community!
Let's first clear up a couple of terms that are often mixed up.
Vivaldi Webmail - the email service we offer to active community members, where you get a YourUsername@vivaldi.net email address.
https://help.vivaldi.com/category/services/webmail/
Vivaldi Mail - a mail client built into the Vivaldi browser. You can add almost any email account you have to it to manage your mail conveniently in one place.
https://help.vivaldi.com/category/mail/
To keep your Vivaldi account active, you need to log in at least once every six months. Doesn't matter to which Vivaldi service.
For example, let's say you actively use Sync and check Vivaldi Social every few days, but don't log into Webmail for 3 years. You'll still have access to your mailbox after not having checked it for years.
Another example, you log in to your mailbox once every 5 months or so. Your account on all our services is still considered active.
Should your account be deemed inactive, you'll receive a couple of reminders to the account's recovery email address before it's deleted. Once deleted, there's no way to restore the account or its data.
https://help.vivaldi.com/services/account/inactive-accounts/