((Get-Mailbox -Filter '*')[0] | Get-Member).Name

# Results
<#
PS C:Scripts> ((Get-Mailbox -Filter '*')[0] | Get-Member).Name
...
RoomMailboxAccountEnabled
RulesQuota
SamAccountName
SCLDeleteEnabled
...
#>

Get-Mailbox -Filter '*' | ForEach {$PSItem.SamAccountName}

# Results
<#
 Get-Mailbox -Filter '*' | ForEach {$PSItem.SamAccountName}
Administrator
...
#>

(Get-Mailbox -Filter '*' -ResultSize Unlimited).SamAccountName |
ForEach{Get-MailboxPermission -Identity $PSItem} |
Where-Object {
    $PSItem -ne 'NT AUTHORITYSELF' -and
    $PSItem.IsInherited -eq $false
} | Select-Object -Property '*'

# Results
<#
AccessRights    : {FullAccess, ReadPermission}
Deny            : False
InheritanceType : All
User            : NT AUTHORITYSELF
Identity        : contoso.com/Users/Administrator
IsInherited     : False
IsValid         : True
ObjectState     : Unchanged

...
#>

(Get-Mailbox -Filter '*' -ResultSize Unlimited).SamAccountName |
ForEach{Get-MailboxPermission -Identity $PSItem} |
Where-Object {
    $PSItem -ne 'NT AUTHORITYSELF' -and
    $PSItem.IsInherited -eq $false
} | Select-Object -Property Identity,User,
@{Name = 'SamAccountName';Expression = {(Get-ADUser -Identity $($PSitem.Identity -split '/')[-1]).SamAccountName}},
@{Name = 'Access Rights';Expression = {[string]::join(', ', $PSItem.AccessRights)}}

# Results
<#
Identity                              User                SamAccountName   Access Rights
--------                              ----                --------------   -------------
contoso.com/Users/Administrator     NT AUTHORITYSELF   Administrator    FullAccess, ReadPermission
...
#>