Digital Postman's interface is a bit on the unattractive side, but it has all the functions you need and they're clear. Scan, send. Very straightforward.
This program is not free, however, and you should only pay for a one-function program if you use that function a lot. If you're regularly scanning documents only to send the scans off, not to archive them, Digital Postman Pro is probably worth the price for the convenience factor alone.