He's provided me customer lists (that include 4,000 old customer emails) that go back to 2007. Many have come and gone over the years as customers and there's probably a good number of emails that aren't good any more and that will bounce.
I uploaded the list to Neverbounce.com and their report said the list is pretty stale (keep in mind that all emails were legit paying customers). I think the list isn't as bad as Neverbounce says, but the ESP will only allow me to upload emails verified by Neverbounce.
I'm thinking of using a "less reputable" ESP that doesn't require such a high level of verification so I can email the list a "Hi, it's been a while since we chatted. I've got something new coming..." sort of email. I would do this to see which ones once and which ones are legit and then export the successful emails list to my new legit ESP. That way the new ESP won't kick us for a bunch of bounces right away
I'm open to any ideas that help me maximize the value of the original list. Can you recommend a less stringent ESP or any other methods to achieve what I'm looking to do?