Create email distribution list

I have never captured my clients and potential clients email addresses. Often clients make contact by email with enquiries. The email I have mainly been using is Yahoo.

I wish to have all clients who have emailed me over the last 4-5yrs (guessing 250 emails) captured in a distribution list for marketing purposes. I would also like the list to be transportable so that in future I can use a different email platform than Yahoo.

I am hoping this is quite simple but please tell me how this can be done in your proposal. If you believe this cannot be done, then please also comment or explain a better alternative that you can provide.

Many thanks.