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.

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Best of luck