From: DeskNow Knowledge Base
http://www.desknow.com/kb/

How can I push mail to my BlackBerry?
http://www.desknow.com/kb/idx/4/093/article/

RIM BlackBerry (tm) devices use a proprietary protocol for email push.

However there are 2 ways of getting emails automatically pushed from DeskNow to your device.

1. Using your mobile service provider

Fortunately, most wireless telephony providers can 'push' email messages directly to your device if the messages are sent to a particular email address.

For instance, at the time of writing of this article, EarthLink will push to your BlackBerry any email sent to userid@earthlink.net . Many other providers have similar arrangements. Check with your provider for more details.

All you need to do in DeskNow is then to create a Mail filter that forwards incoming mail to userid@earthlink.net (or whatever is the correct address in your case). DeskNow lets you also forward messages selectively (ex. only from a certain person/domain, or with a certain word in the subject), so that you can push to your device only the important messages, and leave the others in the main mailbox.

2. Use specialized services that do the 'pushing' from your server

If your phone company does not offer the service described at point 1. you can use one of the many services (some are even free) that get emails from your email account and push them to your device. A popular one is http://www.consilient.com