Try not to send emails to invalid or very old contacts in your database. It's best to run some type of database cleansing tool on your database first. If you send a single message to your entire database and include these dormant contacts you could have a high percentage of bounces. Additionally, old email addresses could be spam traps. Bounces and spam traps will send a notification to the ISP and damage your reputation. The best practice is to send frequent emails to small amounts of old contacts, vs. sending to your entire database or all old contacts, and monitor results. If you're getting lots of bounces phase out this effort. Conduct this "experiment" only after establishing a good reputation.