As the name suggests, a user mailbox belongs to a single person/user; this mailbox is secured with personal access data that only the user knows. A shared mailbox has no access data, but is shared with certain people. Use the Exchange admin center (EAC) or the Exchange Management Shell to create a new distribution group in your Exchange organization or to mail-enable an existing group in Active Directory. There are two types of groups that can be used to distribute messages: Mail-enabled universal distribution. PowerShell provides a powerful and efficient way to retrieve information about a user's distribution group memberships. However, errors and unexpected results often. When auditing mailbox permissions in an Exchange Server organization or Microsoft 365 tenant (Exchange Online), the administrator needs to find all the mailboxes that a particular user has access to. This can be accomplished by displaying person's "Contact Card" in MS Outlook.
[PDF Version]