Sending, receiving, and organizing email in Outlook.com
Hi there! Sorry to hear about what happened on your account. Even though you've changed the password and no longer see unauthorized sign-ins, the account may still have been compromised as the hacker might have left something on your account. I'd recommend checking the account and Outlook settings for anything the attacker may have changed.
First is to check your Security page for unfamiliar email addresses, phone numbers, or security changes:
https://account.microsoft.com/security
Then sign in to Outlook.com at https://outlook.live.com/ and check Settings > Mail > Forwarding and Settings > Mail > Rules. Make sure there is no forwarding address or rule that you don't recognize as this can mess up your emails.
I suggest to try the above first specially if you still have access to your account. Hope the information helps you to secure it!