I am setting up a hosted exchange server for a client, they all use blackberries and need to get their email. I am going to set up POP3 access so they can download mail to their blackberries.

Do I need to provide them with an outgoing SMTP server? or is that something typically provided by the carrier?

I dont want to open up port 25 to my server if I dont have to.

You do not need to provide an outgoing SMTP server.
The BlackBerry network (BIS) will handle outbound messaging from the device.