I also failed to see the 1000 emails for free per month.
I’ve been using their trial plan for 2 years; it costs 1$ for 1000$ emails, with 5000 free emails for the first 3 months.
I believe their “flex” plan was renamed “trial” recently 
A thing bugs me a bit though: Sign Up Free | Mailgun
After your trial, you will be moved to the Foundation 50k, but you can always select pay-as-you-go or a different plan of your choice.
Does that mean that they automatically move us to their Foundation 50k offer (which is 35$ per month), or do they let us choose the “pay as you go” (1$ for 1000$)?
I’ve registered a new Mailgun account on a trial plan recently, in February, for a forum, I’m not an admin of, so I’ll need to pay attention to this.
I believe their “flex” plan was renamed “trial” recently
I quote myself. I’ve been checking on the new account I registered, in the account settings it says: “Plan: Flex Trial”:
Current plan:
But I’ll definitely keep an eye on it. I don’t want the plan to be automatically set to a 35$/month one.