Pick your monthly salary, then imagine how many units of work you would need to sell to achieve the same value at what given price.
Naturally $120 means one sale equals several at $20-$40, thus easier to achieve sustainability, given the ratio of marketing and sales team efforts to effective sales.
We're talking about a mail client here. A $120 mail client whose website mentions not a single advantage over the leading open source solution in its market.
Compared to that, I'd have difficulty believing there are many email clients out there that are actually worse. Even emacs as an email client was better (and it still wasn't great as you can imagine).
I guess sendmail and p̵i̵n̵e̵̵ mutt are good enough™.
It is easier to ask for a random price, without taking the time to understand that there are a set of variables associated with production cost and keeping it sustainable.
All the best to your business.