You have a compule more options:
1. Set up a smartserver/smarthost account with your ISP--most will offer this service, then your mail server pulls and pushes everything to/fromtheir mail server.--this is second best to having a static IP.
2. Use dyndns to receive, and a mail relay to send--dyndns.com had a mailhop service that is pretty reasonable
3. Users can pop email into their exchange mailboxes directly (not recommended--outbound gets flakey)
4. I agree with the other expert: A static IP is definitely the BEST option.