Welcome to the Forums.
The mail your asking for must be accessible from the requesting service. Push email depends on the ability to access the email servers. Google Gmail allows this type of service which closely emulates the BES service used in corporations to send email when received and allow contact and other similar services to be synchronized. If your email service is open to the internet and you want to set up a small BES service for your business I would check the RIM web site. I included some links for the info.
If not you can check the BIS service abilities and see if it meets your needs.
http://www.blackberry.com/btsc/micro...0%201125333048
BlackBerry - PDA Software Downloads - Support & Services at BlackBerry.com