Add-PSSnapin *Exchange*

$mailboxes = (Get-Mailbox -ResultSize Unlimited).PrimarySmtpAddress

Foreach ($mailbox in $mailboxes){

$now = (Get-Date)
$reportemailsubject = "Dummy Email"
[String]$Body = (Get-Content "C:\Scripts\DummyEmail\longtext.txt")

$smtpsettings = @{
    To =  "AllUsers@PS.local"
    From = "$mailbox"
    Subject = "$reportemailsubject - $now"
    SmtpServer = "192.168.1.113"
    }
    
Send-MailMessage @smtpsettings -Body $Body -BodyAsHtml -Encoding ([System.Text.Encoding]::UTF8)
}