Most likely because they do not have a valid electronic address for the mail
transport you are using. Unless you tell us the mail account type, how you
are selecting the recipients and how you have validated the email addresses,
we can't help further.
"Martha" <Martha@discussions.microsoft.com> wrote in message
>I send e-mails to legittimate addresses and I get a system undeliverable
> message saying that some or most are invalid recipients. why?