Users with administrator rights (an option is visible in the Employee profile) can create, edit, delete, change passwords or attach mailboxes to Bitrix24 account users with either an @bitrix domain, or a corporate one.
Use Add mailbox option under Settings section on Manage E-Mail Accounts page to add new mailbox. In a new Mailbox creation form you need to add mailbox address (name) & password.
You can create mailbox for a particular employee directly with the help of Create option next to employees name.
Select another mailbox option allows to attach one of existing mailboxes to the user.
Edit, change password or delete mailbox
Use Edit option when you need to attach another mailbox to the user or create a new one for him\her.