I really like Thunderbird. I have about six different email accounts that load there, all POP, not IMAP. I have rules in place for sorting mail when it comes in (there are advertisers I don't want to cancel but they fill up the regular mail, like JOANN Stores. Create a rule telling those posts to sort to that sub-folder, it's done.)

That said, Gmail has a lot of interesting functionality on the website, but I go in periodically and sort and dump most if it. Since I have copies on my computer I prefer to store it there. Who knows what else Google is doing when it crawls your email folder with various algorithms for spam and such.