Hi Domini,
Can you please check your Sent Items folder and remove any filters. Make sure your account is fully synced: File > Account Settings > select account > Change > More Settings > Advanced and confirm all folders sync. Use Search with “sent:” plus the year to find emails.
If still missing, check Deleted Items or Recoverable Items (Folder > Recover Deleted Items) or sign in to Outlook.com to see if emails exist on the server.