“If you need to send to an ‘old’ list, say one that you haven’t emailed in over six months, or a purchased/rented list, it’s best to have that list cleaned by a list hygiene service.”
Use an email hygiene service such as Fresh Address before sending emails to old list recipients. This removes emails deemed undeliverable, spam emails, and the emails of subscribers who continually complain about spam issues. Do this with any list more than six months old.