According to the last several clients I’ve talked to, I don’t believe the mailgun flex plan is still available. If you need to send more than 100 messages per day, it costs $15/month.
The issue with port 587 being blocked is a digital ocean issue. I’ve had several clients who just stopped being able to send mail because digital ocean started blocking the port without warning. It’s a trivial fix to switch to port 2525, though.