Mozilla Thunderbird is supposed to be the most advanced and full-featured free email program, but I am baffled at how such a program could be so badly designed. Here are the main things I don't like about it.
It is completely unusable if you have a large mailbox and a slow connection. It insists on downloading your entire inbox, whether you like it or not.
The defaults are destructive. If you set up a new email account on it, it will immediately start deleting messages from your mail server. Yes, you can change the settings to stop it, but the defaults should be non-destructive.
It is extremely tedious to setup new email accounts. You have to manually process dozens of menus, filling in lots of obscure settings, and there is no way to automate it. If you want to install Thunderbird on another computer, you have to do it all over again.
It allows you to separate your incoming and outgoing accounts, but it is extremely confusing. Eg, if you setup an incoming account, the first thing it does is force you to give a name for your outgoing messages. It would make more sense to set your outgoing settings when you setup your outgoing account.