You can access webmail for an email account under your domain name by putting the word "webmail" after your domain name in your browser's address bar, like this https://www.mydomainname.com/webmail/.
When prompted, enter your full email address and its password in the appropriate boxes and then click Login. You may choose one of two webmail interfaces, which are available in the drop-down box above the Login button before logging in.
Please note: You can only access webmail on accounts that are Mailboxes. Aliases and Groups, which are forwarding addresses, and Autoresponders are not Mailbox accounts.