Just wanted to quickly post this link, which describes the differences between publishing sandboxed vs. admin approved solutions.
I hadn't realized that sandboxed forms can't use email submit connections!
From the MSDN page:
Publishing Form Templates as Sandboxed Solutions
Publishing a form with code as a sandboxed solution is no different from publishing any other form to a document library. Just use the publishing wizard as usual and your form will be uploaded to the server and will operate in the sandbox.
Note that there are certain restrictions to deploying your form as a sandboxed solution:
Must be an InfoPath 2010 form.
Must use C# or Visual Basic as the programming language.
Cannot submit to e-mail data connections.
Cannot have properties promoted for part-to-part connections.
Must not have any managed meta-data controls or data connections.