I have been stuck on my hosting provider's webmail portal (Roundcube) ever since the macOS Sierra switch. Mostly because I don't want to go back to Mail.app. I have no idea where to look for a solid IMAP client though.
Did they ruin Mail.app? I remember it being fine for ages but I haven't used it for some time (I use FastMail's web interface). Perhaps you could try Thunderbird? Or look for an alternative here: alternativeto.net/... and on the App Store?
It's not that they ruined it, I was just never completely happy with its reply interface. It also did weird things with text-only email, which is my preferred way to write email. There are of course a million email clients available, as the AlternativeTo link suggests. Far too many to compare. But I'll figure something out. I am starting my foray reading thesweetsetup.com/...