I've used Thunderbird for years. I keep a huge number of emails. I move them to folders to organize or archive them, but I don't delete anything so I have hundreds of thousands of messages.
I like the way TB is conceived, and the way it works as long as the volume of data is small. But it just doesn't scale.
It has a lot of ridiculous design flaws such that, for example, any time consuming operation blocks the whole UI completely (and you don't even know for how long) as if everything was implemented in a single monolythic all-tasks-are-blocking way. I'm tired of it.
So what is the alternative that you would recommend as an email client program with all the usual basic features one expects from any email client program? Important: I mainly use POP3, much much more than IMAP, and my main account is on gmail.
This question is not intended to be a rant against TB (I admit it is, as a side effect); I have highlighted its weaknesses BECAUSE the answer I'm looking for is a recomendation for a program that doesn't suffer from these issues.