Digital Postman Pro is a straightforward program that allows you to put a document on your scanner (or load one from your hard disk), and then email it.
While the hard disk use is of secondary utility - it's as much work to use Digital Postman Pro as it is to use your mailer, so why bother? - the scan-and-mail function is a serious time saver.
Digital Postman Pro'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.
Digital Postman Pro 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.