Recommended bandwidth for email hosting?

I have 23 users currently on an Exchange 2007 server. I have a POP connector pulling mail to the Exchange server. I’m getting complaints about mail coming down in batches – which it does. I had the POP connector set to pull down every 5 min, but that’s not good enough for them. So I now have it set to 1 min. Eventually, the mail host is going to get tired of my system hammering them for e-mail and will block us temporarily.

My question is this:

If I have 23 users on a server, how much bandwidth should I have at this site to handle all the users?