If you don’t need cross-platform, there’s usually something better native to your platform of choice.

I’m preferring Evolution over it right now for Linux.

If you don’t need to deal with multiple email providers, your email provider might even have a better app. I’d rather use the FastMail desktop app but I have other email accounts with other providers.

A big pet peeve of mine is how Thunderbird is so bad at exporting your profile settings to multiple computers.