The "Fast Lane" Answer
SmartyStreets provides address validation, and our core data comes from the US Postal Service. Since "care of" information is not pertinent for address validation, it is moved to the address line 2 field. This will always be the case. Any time you submit information in address1 or address2, only the valid address delivery data will be used in address1.
The best way to handle "care of" or other extra address data is to have an additional field where your customer can enter specific delivery instructions.
The "Scenic Route" Answer
"Care of" information is treated in the same way as any other "extra" data would be. Suppose you submit an address like this:
address1: leave packages behind bushes on the left
address2: 1402 West Center St
The "extra" data would be preserved but shifted down to the address2 field, where it belongs, and the output would look like this:
1402 W Center St.
Leave packages behind bushes on the left
Orem, UT 84057
Similarly, "care of" information is not part of the valid address, so it is moved to the address line 2 field. So this . . .
address1: Care of Xavier Institute for Higher Learning
address2: 1407 Greymalkin Lane
city: Salem Center
. . . would look like this:
1407 Greymalkin Ln.
Care of Xavier Institute for Higher Learning
Salem Center, NY