You should all consider the best email apps out there, many exist now, but make sure you look at Terms for all of them. K9 is open source and a very good client. Our app which has been around since 2010, MailDroid is ad based and an ad-free version which you can pay for. Many others are not clear on what they do. For example, AquaMail was bought by Mobi and comments have shown tracking was added in. TypeMail and BlueMail and others have no open revenue model, but all your emails pass through their servers. Do they sell your data?
Some people don't care, but many should. If you look at the recent exposure of the free email spam unsubscriber (unroll.me), you will see what happened. While many people know that a free service needs to make money somehow, an open show of how they make that money is important. If you care about this, then you should be sure to look at what Terms exist for each mail client.