Packages "Domain + email" or purchase of "Email" upgrade.
Login from here with your login data, click on "Active domain management" > "Panel" > "Email management". There you will find inboxes data.
Packages "Linux basic/plus hosting" and "Windows basic/plus hosting".
Email management it's only possible through Plesks control panel. For privacy, Plesk doesn't admit to see passwords, but only replace them.