Sent Mail Returned as Undeliverable

I just got a new computer with Outlook Express installled. It works fine.
This morning I updated Windows to Service Pack 3 and it installed Outlook,
which I like better and would rather use. It works fine receiving mail, but
all the mail I send is being returned as undeliverable. The message is that
none of my email accounts can send to the recipient. This happened with four
different sent emails.

--Ruby