Corporations steal from us all the time, and they don’t even let us buy their content, they only sell limited access that can be revoked at any time (see https://lemmy.dbzer0.com/post/9531016 or https://lemmy.dbzer0.com/post/12904663). Under these circumstances, there are no ethical issues with piracy.
Nothing about the program itself is subscription based. All of the normal features of an email client (that you would also find in Thunderbird) are available for free. You only need to pay if you want to use their services like Send later, read receipts or link tracking, because these requires backend servers and actually costs the money.