I've recently used 3 mail clients: +Thunderbird +Claws Mail +Geary

Neither of them was "good enough" for me. You may think that I'm spoiled, but in fact I just want a better mail client, and I want to have it, by eighter configuring one, or helping with develpment. First of all, I don't like web-based mail clients, so be aware of that. Don't try to covert me, I've made my choise.

Our first candidate is Thunderbird

One of the best clients I've ever used. It has smart autocompletion of names, the address book build itself as you send mails to people, it's nice looking. The cons - long startups, and big memory footprints.

Claws Mail

the lighweightest of them all. Quite ugly, but with a new set of icons, you can deal with it. Autocompetion is crap, and is one of the hardest to set up. Annoying stuff, like the pop-up, when sending messages, or when CM can't find network connectivity. The one which shows up when sending mails can be disabled. The address book can be imported from folders, but you have to know it. TB does it automagically. Another annoyance is that, after inserting the receiver email, going I have to press TAB twice before I can write the subject. I know it's a small annoyance, but such little things makes the thing harder to use.

Geary

It's new, elegant,(but is kinda ugly on my setup, with awesome wm, I think I should use gnome-setting-daemon for setting gtk3 themes...and other stuff) I've used it the least, but autocompletion seems fine, the interface, while it lacks the amount of configurability of the other 2, is pretty well set. It's resource hungry, it eats tons of memory, and it contantly eat some processor, while the TB and CM seems to sleep. But I wont be too harsh with this one as it's the youngest (Geary hasn't reached version 1 yet).