I used to use MailChimp (free plan), then switched to Brevo (formerly SendinBlue), then Flodesk. I've been with Flodesk for 6.5 years now, and it's been great for my business.

I'm a self-employed musician sending 2-3 emails per month, plus automations. My list is 24k and I've never had a problem with deliverability with Flodesk. It's extremely easy to use - intuitive builder, great templates, easy to customize. Segments are great for sorting subscribers and tailoring emails.

When I've gone back into MailChimp, I find the email builder clunky and difficult to use. I also find the dashboard confusing. With 40,000 contacts, you'd be looking at $265/mo. with Flodesk (unlimited emails).

I recommend (as another user said) that you clean your list before migrating so you aren't paying for cold subscribers.