I assume you are talking about Thunderbird? I used it professionally for two years before deciding it was much, much worse than the alternative (Outlook webmail + a gross CLI tool for other misc email). It's slow, it uses a lot of disk space for bad reasons and just generally operating on a lot of email is a massive pain.

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).