Can I send to my entire email list using the E-Newsletter Plugin?

I have a list of dedicated, double optin subscribers that I’ve worked really hard to collect over the years. There is about 23,000+ subscribers now on my list. If I were to switch over from iContact to using the ENewsletter plugin, how am I able to send out to all 23,000+ of them each week? My SMTP relays only can handle 250/day. Thoughts? Thank you!

Also, would they have to optin at any time AGAIN if I decide to start using the Enewsletter plugin?