Karen, who or what manages the SMTP server you're using? If it's a cheap/free one you may find there's a limit to the number of recipients per email and/or the number of allowed sends per day. If you've exceeded either, you won't be able to send any more. Easy enough to test: try to send an e-mail manually from the MAPI client on the test machine. Just an idea to investigate.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
