Can I send AIP protect mail to a company uses on-premises exchange server？

You can send to anyone, the question is will they be able to open it :) That part depends on the way you've sent the message, if using standard templates/labels, then the answer will be no, as the user from company B must authenticate against Azure AD in order to get the relevant cert to view the content.

If you are using the new OME however, you can send to any address and they can get to the message even without an Azure AD account provisioned. Read here: https://support.office.com/en-us/article/office-365-message-encryption-ome-f87cb016-7876-4317-ae3c-9169b311ff8a