I just posted a new build of Maildrop that has a work around for the error with attachments on Lion Mail. (do check for updates from the menu)
To recap the story so far:
1. There is a bug in the version of Mail provided in OSX 10.7 (aka Lion) that causes an error when you try and access an attachment via applescript, the error shows in maildrop when you try and add an email that contains an attachment.
2. The new build will skip all attachments applescript operations if you turn off attachments in the maildrop preferences, This will allow you to add emails as normal, but not any attachments.
3. When apple releases a fix for the applescript problem, you can re-enable attachments in the preferences and be back to normal. (there is a bug logged with Apple on this, i will post updates when i get them to the issue tracker)
4. When you start the new version of Maildrop on Lion, there will be a window with this information as well.