To access your Webmail, you'll need to log in to your Iowa-Host Account Portal.
Once you are logged in, go to the top left box that says "Your Active Products/Services" and click on "View All".
On the next screen, click on the green "Active" button for the particular domain's Webmail that you want to login to and it'll take you to shortcuts for that domain.
Once you're on the screen with more options, you'll see a list, on the left side of the page, where you'll have options that allow you to go to cPanel, Webmail, or allow you to Change your Password.