In a Web browser, visit the URL webmail.domain.name, where domain.name is the Internet address of your website. When prompted, specify your full email address as the username (for example, firstname.lastname@example.org), and specify the email address password.
When logged in to Plesk
panel, go to Mail, and in the list of email addresses, click the icon corresponding to the email address you need.
Also, make sure that a webmail solution is enabled. Open the Mail section, then the Mail Settings tab, click the name of the domain for which webmail is inaccessible, and select a webmail client in the Webmail menu.