Excellent K-9 mail app for Android keeps your messages on a leash

Google's conventional e-mail client for Android has always felt like a second-class citizen compared to the company's GMail application. It has a very limited user interface, lacks basic features like support for moving messages between folders, and isn't particularly reliable. Google has been slow to address the program's weaknesses and doesn't seem to notice most of the complaints.
Fortunately, there is a good third-party fork called K-9. It's not particularly pretty, but it's highly functional and well-maintained. K-9 is based on Google's original Android mail client and is similarly distributed under the open source Apache license, but it's got a whole pile of additional features.
K-9 supports IMAP IDLE for push messaging. It also gives the user fine-grained control over push and polling message retrieval behavior, with various configuration options that can be set per account and per folder. It also gives you a bit more control over notifications, including the option to have a separate notification tone for each account. You can also configure a "quiet time" range during which you don't want any intrusive notifications.
The K-9 message list view is a lot more sophisticated and configurable than that of the default mail client. You can, for example, configure it to show a short excerpt of the message text. You can also hide the selection checkboxes and the star icon that is used to mark favorite messages. When the checkbox is hidden, you can still select a message for a batch operation by dragging it to the right in the list view—making the checkbox appear temporarily.
You can individually adjust the font size of many different user interface elements across the account list, folder lists, message lists, and the messages themselves. There are also several different options for the date format that shows up in the message views. K-9 has two different "themes" which basically allow the user to choose between a light color scheme and a dark color scheme.