There are two possible solutions that come to mind. Unforunately they are going to depend on your business requirements, scope, and infrastructure. All the same...
1. You could create a LiveCycle service that submits the form using the EmailService. To do this, you would probably have to submit the form data to the service via a SOAP connection, merge it into a "document" process variable with an empty copy of the PDF/XDP stored on the file repository or server (it it's already stored there, great!), then use the EmailService to send an email with the "document" variable you just created as an attachment. This will prevent the user from having to even open their email client and click send.
2. The other option is to let your users know that they can probably configure their Outlook, Novell, etc. to utilize their Gmail, Hotmail, or whatever account. I have Outlook using my Gmail account and most webmail site will provide you directions to configure that. Again, not the greatest solution, but a possible one.
Ryan Lunka
Cardinal Solutions Group
rlunka@cardinalsolutions.com