Yep, you've got the right idea. The by time you get to your Select call, you're looking at a mailboxStatistics object, which has no PrimarySmtpAddress property. How you deal with this depends on whether you're running PowerShell 4.0 or not. In PowerShell 4.0, they added a new common parameter called -PipelineVariable which can help this situation with a "one-liner" approach like this:
Get-Mailbox -ResultSize Unlimited | where {$_.PrimarySmtpAddress -like "*@domain.com"} -PipelineVariable mailbox | get-mailboxstatistics | where {$_.lastlogontime -lt (get-date).adddays(-90)} | select @{Name = PrimarySmtpAddress; Expression = { $mailbox.PrimarySmtpAddress }}, LastLogonTime
If you're running an older version of PowerShell, then you don't have -PipelineVariable, and have to resort to a loop with a nested pipeline instead. The performance isn't quite as good, and the code doesn't look nice on one line anymore, but the idea is the same:
Get-Mailbox -ResultSize Unlimited |
where {$_.PrimarySmtpAddress -like "*@domain.com"} |
ForEach-Object {
$mailbox = $_
$mailbox | get-mailboxstatistics | where {$_.lastlogontime -lt (get-date).adddays(-90)} | select @{Name = PrimarySmtpAddress; Expression = { $mailbox.PrimarySmtpAddress }}, LastLogonTime
}
