I just ran some rejected addresses. What does that mean?
If I were you, and I never got any rejected addresses, I'd frankly worry that SmartyStreets isn't doing what it says it does. In other words, this is perfectly normal.
Now, your next question of course is going to be "Well, how many rejected addresses is too many?" To answer that, you really need to think about where you are getting your addresses. Are you getting addresses directly from your clients or are you scraping addresses from shady websites? 5% rejected may be pretty high in the former case, while in the latter, 80% rejected may be wishful thinking.
So, what precisely is a rejected address?
Well, from the standpoint that a rejected address is one that is not deliverable by the USPS, an address can be rejected for several reasons. Very common are addresses that are good USPS addresses (AABB) but are currently either vacant or not active and thus are not deliverable. Also very common are addresses whose street/city/state/ZIP combination is correct but the primary number is incorrect (AAM3).
Here's a good one: 206 Ruth Ave, Severence CO 80546. Google has a picture of the house and the house is clearly marked '206'. This must be a valid address! Why was it rejected? The answer is simply that all mail in this ZIP area is delivered to PO boxes ONLY. The address above is neat and tidy and is a good physical address but is NOT a valid USPS address. This is why it was rejected.
Other causes may be more obvious even if you don't catch them before processing your list. Sometimes the primary number gets left off the address like: Reed St, Lakewood CO 80226, or PO Box Holt MN 56738. Sometimes the address given to you was just plain bad (A1)- meaning someone gave you an address where the street given can't be matched to the city/state/ZIP combination.