When an address is updated via AddressFinder, the details of the original address that was matched, such as the do not mail flag, are copied over or retained on the new address.  A record may have an address that is marked "Do Not Mail" and when a new, mailable address comes back and that Do Not Mail flag is retained on the new, valid address.  

This because the Do Not Mail flag may not necessarily just indicate that the address is undeliverable.  Instead, for example, it may also mean that the address should not be mailed to by preference of the donor, which in that case, would apply to the new address.