Hi
This is a good question.
I think that the best use of your old email list depends on "how" you got the emails + "why" people gave them to you (at the time). Examples: did they buy a product? Sign up to a newsletter? Contact you?
Based on "how"/"why" you got the emails, you can then reach out to them by providing them value (giving them something related to what they wanted when providing your email).
For example: if someone bought a product, then it would be ok to offer them similar or complimentary products. If someone subscribed to your newsletter, send out an informational email (and perhaps gently include 1-2 products/services that you are offering).
Important notes:
1. Remember to utilize the email you are sending out to its' fullest - invite them to become members, subscribe to your blog, or whatever other method to keep them engaged. You only have on shot.
2. The email should not be spammy - respect their time.
3. Allow for a clear and easy unsubscribe method and unsubscribe anyone who asks.
4. Use 'cool'* and friendly wording (and stay short and to the point) (* unless it's health products/services etc).
5. As for the open rate, this is a whole huge subject and not for this answer.

I've successfully helped over 300 entrepreneurs, startups and businesses, and I would be happy to help you perfect the above advice. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david

Best of luck

LinkedIn will allow you to sync emails with the network. Are the leads professionals? If so request a call with me and I can show you how to do this. Full disclosure LinkedIn will suspend your account if you get too many declines. Cold emails are not recommended. Warm or double-optin will work best.

Yes you can connect with the social media account when you search email on the social media and find the social media account associated with the email.

I note, that you prefer not to polute your new solution. That will reduce the subscription count, but if you get better leads, then that is fine.

One way would be to run a campaign on Linkedin or Facebook (depending on what you know, and where your audience is). There you can upload the single opt-in mailadresses, and campaign towards them. And then support that with a landing page with signup to your new environment.

You could also do "twin-identification" (at least on Facebook), where Facebook will match your audience with similar prospects based on demographics and interest.

You could also just import the 10.000 emailadresses in your new environment and give them a specific tag. Then you can easily delete them again, if they just continue to perform in the same way as they have always done.

There a many ways to do it, so good luck with your implementation. It would be a shame to just lose them.

If you want to, let's set up a call to discuss this further.

Best regards
Kenneth Wolstrup