Trying to impliment email distribution.

We are a small company running NT server 4 with 20 clients. We have set up a domain (abc.com)with our ISP. We have several mail boxes (admin@abc.com) (service@abc.com), and each user has an email address (first_lastname@abc.com)each assigned to a maiolbox. Our managers currently log onto the ISP mailserver to download from thier mailbox and manually distribute to each user. Is there a cost effective, automated solution to this?