You can access Webmail by browsing to 2096 of your primary platform name, for example:
https://xx-platform-xx.wpopt.net:2096
At this page you can then enter your email address and password to log in.
You can alternatively access Webmail through cPanel:
1) Log in to cPanel (instructions here)
2) Click on "Email Accounts" under the "Email" section:

3) Scroll down to your email accounts and click on "More" next to the email account you wish to access, and then click on "Access Webmail". You will be automatically logged in to this account.

4) The first time you access webmail on an account, you will need to choose which webmail application to use. Roundcube is by far the most modern and visually appealing client, so I recommend this one:

5) Click on "Set as default" and then you will be automatically logged in to the email account.
