#Firstly, get the users mailbox... ONLY objects that are UserMailbox and not HiddenFromAddressListsEnabled
$MailboxUser = Get-Mailbox | Where {($_.RecipientType -eq "UserMailbox") -and ($_.HiddenFromAddressListsEnabled -eq $False)}

#Secondly, get the mailbox statistics for the selected users. Some formatting also
$MessageBody = $MailboxUser | Get-MailboxStatistics | Sort-Object TotalItemSize | Select DisplayName, ItemCount, @{Expression={$_.TotalItemSize.Value.ToMB()};Name="MailboxSize MB"} | ConvertTo-html

#Finally, enter your mail server details
$FromAddress = "FromAddress@YourDomain.com"
$ToAddress = "ToAddress@YourDomain.com"
$MessageSubject = "PowerShell Report Subject"
$SendingServer = "YourMailServerAddress"
$SMTPMessage = New-Object System.Net.Mail.MailMessage $FromAddress, $ToAddress, $MessageSubject, $MessageBody
$SMTPMessage.IsBodyHtml = $true
$SMTPClient = New-Object System.Net.Mail.SMTPClient $SendingServer
$SMTPClient.Send($SMTPMessage)

#Written by Matt Shadbolt
#http://www.mattlog.net