A hacker put a forwarding rule on my outlook email that I can't delete

If the rule is in Outlook.com or Outlook on the web, remove both the inbox rule and any account-level forwarding, because either one can keep forwarding mail.

1. Sign in to the mailbox in the web experience.
2. Select the gear icon, then go to Mail settings.
3. Open Mail > Automatic processing > Inbox and sweep rules.
4. Update, turn off, or delete any rule that forwards messages to an unknown address.
5. Then open Mail > Accounts > Forwarding.
6. Select Stop forwarding and Save.

If the rule shows as broken or editable in classic Outlook, use:

1. File > Manage Rules & Alerts.
2. If Outlook reports a broken rule, select OK.
3. Select the rule marked with an error.
4. Select the links under Rule description to edit it, then select OK.
5. To change or remove a normal rule, select the rule, then use Change Rule or Delete.

Important checks:

• If forwarding is configured to multiple addresses, the rule might stop working if the account exceeds the allowed number of addresses.
• Forwarded copies can remain in the mailbox and still count against the mailbox limit. If the mailbox is over limit, forwarding rules might not work until enough mail is deleted.
• Suspicious forwarding rules are a security risk because attackers can use them to hide, delete, move, or forward messages.

If the forwarding keeps coming back, check for any other forwarding setting in the mailbox and remove unknown rules again in the web settings first.

References:

• Fix NDR error "550 5.1.10" in Exchange Online
• Classic Outlook
• Outlook.com
• Alert classification for suspicious email forwarding activity