You must make sure the post office will be able to deliver your mailing. The problem is you don't know which addresses are bad. Here is where a good mailshop can help. Your database can be checked against a CASS certified database. At the same time, the software will standardize the address to meet postal automation requirements.
Many businesses rent mailing lists.
If you are going to enhance the list, and you want to spend your total project budget, then order more names than you plan to mail, and mail only those addresses which are accurate and standardized. If you want to reduce your project budget, order the original quantity you were going to mail. Enhance the list and mail only qualified names. Whether you love it or not, you will need to factor the post office into your planning at the beginning of your project. While automation is improving performance it has some restrictions.