This is probably not a typical bid, but...
I've been around MailChip since MailChimp has been around. Sent over 200+ campaigns for various clients (also with Campaign Monitor, Constant Contact...), did email structure/copy consulting, CTA positioning, design layout, A/B suggesting, etc.
From the idea part to the part of executing, I've optimized emails to the point where they actually get delightful. I did several importing/exporting of users/emails in the past so that shouldn't be a problem, no matter how big the list in the back is.
2 things I also want to point out:
1) I'm new here, so forgive me for not having any qualifications listed (I see others do)
2) I'm looking for anything that goes beyond a typical one-occasion task and see the whole process of email marketing activities as an opportunity to help you get the maximum ROI, long term.
Let me know if you have any questions!
Thanks ;)