new mail-account automatically forwards external mails to internal address

Use Exchange Online PowerShell to check mailbox-level forwarding:

Get-Mailbox <user> | FL ForwardingAddress,ForwardingSmtpAddress,DeliverToMailboxAndForward

If either forwarding address is populated unexpectedly, remove it with:

Set-Mailbox <user> -ForwardingAddress $null -ForwardingSmtpAddress $null -DeliverToMailboxAndForward $false

Also check inbox rules with:

Get-InboxRule -Mailbox <user> -IncludeHidden

Disable or remove any unexpected forwarding or redirect rules.

If both checks are clean, review Exchange admin center under Mail flow > Rules. Also review the audit log for Set-Mailbox, New-InboxRule, or Set-InboxRule activity.

Unexpected forwarding can indicate unauthorized account activity, so if you find anything suspicious, I’d also review recent sign-ins and secure the affected account.