A user on a Cloudron installation can have access to one or more mailboxes on the same server. To do this, locate the webmail application of your Cloudron installation on your Dashboard, for example https://webmail.9wd.eu.
The username should be the email address you use to log in, for example info@domain.com. And then as password use the password of your Cloudron user.
A mailbox can be shared by several users through groups created in Cloudron. When a group is assigned as the owner of a mailbox, all members of the group can log in to the mailbox.