Sometimes I have been able to email invoices with no problem. Other times, the appropriate screens appear, including an Outlook Express box asking if it is okay to send the message. Although I click "Send", the message does not appear in Outlook Express as sent. When I send from the queue, same problem.
I can't see any difference between those that were sent and those I am unable to send. In my firewall, Adagio Invoices is permitted to send mail. Is there any other utility that might have to be enabled for Adagio to send mail?

But then why would it work sometimes and not others?

I tried sending one of the messages to myself, and it worked. So it seems to have a problem with certain email addresses.