Run PowerShell cmdlet:
Get-Recipient -ResultSize unlimited
This list can be filtered with the -RecipientType parameter with the following switches:
ArbitrationMailbox
ConferenceRoomMailbox
Contact
DiscoveryMailbox
DynamicDistributionGroup
EquipmentMailbox
ExternalManagedContact
ExternalManagedDistributionGroup
LegacyMailbox
LinkedMailbox
MailboxPlan
MailContact
MailForestContact
MailNonUniversalGroup
MailUniversalDistributionGroup
MailUniversalSecurityGroup
MailUser
PublicFolder
RoleGroup
RoomList
RoomMailbox
SharedMailbox
SystemAttendantMailbox
SystemMailbox
User
UserMailbox