There's also Sylpheed (https://sylpheed.sraoss.jp/en/) and Claws Mail (https://www.claws-mail.org/).
If I were just using email with developers I would not care about sending/receiving only text emails, but unfortunately the real world is a bit more complicated than that...
The tagline "for busy professionals, like you." immediately made exit the page without thinking.
It could be the best product in the world but I'm not going to use a product that treats me with such a patronizing attitude.
And whatever genius decided to hide the pricing behind another page doesn't understand their market. The price is the first and only thing I care about if we're talking about paid desktop clients. Everything else is secondary. And they might as well put the big honking "$40 per license" pricing right there at the top instead of further down the fishing rod. Anyone who considers that a worthwhile deal is going to pay it anyway. It's not a bad deal for a lifetime license.
However many customers would probably like something more than a 30-day free trial without a lifetime commitment. Offering a $3/mo subscription would probably majorly boost sales.