There are many restrictions on submit by email and they vary by the code used to perform the submit. If you use the builtin "mailXXXX" method the user must have Windows and and a MAPI enabled email client. And if you use the URI "mailto" format the user with Reader must be viewing the PDF from within a web browser. And then you have the timing problems with slow computers or larger PDF forms. And this does not even start to cover email filters, identity issues and son on.
If you can use the sumbit to server script.