Dear Riccardo Del Monte,
Welcome to Microsoft Community.
I understand that you got your account back after it was stolen and changed your password, but he added security information to your account.
If the hacker has added security information to your account, then this is dangerous and you need to change this security information to your own.
You need to go to https://account.live.com/proofs/manage/additional to change the security details, enter your account name and password, and when verifying your identity, select “Show more verification methods” and you will see all the security details. If none of these belong to you, you will need to select “I don't have any of these” to reset the security information.
Follow the prompts to add the new security information to your account and after 30 days you will be able to verify your identity using the new security information. If you don't have two-step verification turned on for your account, you can visit https://outlook.live.com to check your email now.
What does “Security info change is still pending” mean? - Microsoft Support
Please scan your device fully with an anti-virus software to make sure that your device is safe.
After making sure that your device is safe, you can log on to https://outlook.live.com and click on the gear icon in the top right corner, find “Mail”, check “Rules”, “Junk mail”, “Forwarding” for unfamiliar rules and contacts, auto-forwarding is turned on, etc.
Read the following links for information on how to strengthen your account security.
How to help keep your Microsoft account safe and secure - Microsoft Support
Feel free to post back if you need further assistance.
Best Regards,
Jason.YA - MSFT | Microsoft Community Support Specialist