Is it possible to disable Junk Mail filtering in mail.app

Yes — by selecting "Perform custom actions" and then clicking "Advanced," you can configure Mail's internal rule that determines both what is considered junk mail and what Mail should do with it. Add a condition to this rule that reads "From does not contain @example.com", and no such mail will be considered junk.

As far as I know, this is the only mail rule that can determine whether something is marked as junk or not. Ordinary rules can only move messages into or out of the Junk folder.

Select the email then using preferences->rules will fill in the values from theemail in the selections e.g. if you choose From it will show who the email is from.