Bill:
You send an email with an attachment from OE and you can see it in the Sent folder still with the attachment. The client gets the email but with no attachment. And if you go to the Sent folder and re-send the email, it goes through successfully, attachment and all?

It seems as if Outlook is treating the first email differently because it comes from another application. Look at your security settings in Outlook to see if there is anything about handling of emails generated by external applications. Adagio hasn't changed but Outlook and the environment have so I'd look there next.

When your customer opens the initial email (without the attachment), is there any message indicating that the attachment was removed?