Gets UPN, Mail, and primary SMTP address (from proxyAddresses)

Get-MailAttributes.ps1

# Show all
Get-ADUser -ldapFilter '(msExchMailboxGuid=*)' -Properties mail,UserPrincipalName,ProxyAddresses | 
  Select UserPrincipalName,mail,@{N="PrimarySmtpAddress";E={($_.proxyAddresses | Where {$_ -clike "SMTP:*"}).Split(":")[1]}}
  
# Show only where there is a mismatch
Get-ADUser -ldapFilter '(msExchMailboxGuid=*)' -Properties mail,UserPrincipalName,ProxyAddresses | 
  Select UserPrincipalName,mail,@{N="PrimarySmtpAddress";E={($_.proxyAddresses | Where {$_ -clike "SMTP:*"}).Split(":")[1]}} |
  Where {
    ($_.mail.ToString() -ne $_.userPrincipalName.ToString()) -or
    ($_.mail.ToString() -ne $_.PrimarySmtpAddress.ToString()) -or
    ($_.UserPrincipalName.ToString() -ne $_.PrimarySmtpAddress.ToString())
  }