“Verify your older lists with a third party service such as Kickbox or Briteverify to clean out inactive or erroneous addresses before sending. The nominal cost of cleaning your list is preferable to damaging your sending reputation and will help ensure that your list is best positioned for success.”
Better safe than sorry when reestablishing a relationship with a list that hasn’t been communicated with in a while. Avoid sending to addresses or lists older than 2 years. Scrub through the list to ensure bad emails are deleted and clear out inactive users to avoid any penalties from your ESP.