What proof will I have that the USPTO has received my submission?

Once you have submitted your documents electronically, you will immediately receive an Acknowledgement Receipt, which contains critical information, including a time and date stamp of receipt, a listing of the files/documents associated with your submission, page counts of those files/documents where applicable, an application number and a confirmation number. This electronic receipt may be printed and it carries the security and authority of the Postcard receipt commonly provided in paper-filed applications and returned to you by the United States Patent and Trademark Office acknowledging receipt of your documents.

USPTO FAQ - Questions and Answers (07JUL2007)