We have Port 25 open for POP/IMAP connections. There are a couple of options you can pick from if your ISP has blocked port 25.
Upgrade to a full exchange account. You can use Entourage on the Mac or Outlook for PC's to connect securely to our network completely bypassing the ISP's restrictions.
Access email online in our Outlook Web Access (OWA) interface at http://mail.saashost.net and hovering over "Log In" to select the appropriate platform - this will work just fine but is not as convenient as an email client on your computer, and you may not be aware of which Exchange platform you reside on.
Use the alternative outbound SMTP port, which is port 587.