If you control the DNS for your own domain, you can point just the mail exchanger (MX) records at Pobox. The MX servers for pobox are:
These should all have a priority of 10. Leave the host (or hostname) field blank. If your provider requires an entry in this field, please put @.
Once you change your MX records, all mail for your domain will begin being sent to Pobox. So, if you have any users at your domain, please make sure you set up forwarding or mailboxes for them before changing your MX records.