Here's a tribute to complexity once in a while:
On my mobile device my favorite eMail client is Profimail. Yes, it's little known but it is ultra configurable and does exactly what I want. It makes an LED on my phone blink when eMails come in, I can use POP3 and not IMAP because I like that, I can configure when and how messages are deleted on the mobile and on the server, I can configure how much of an email is downloaded in the background so I don't have to wait when reading the email later because only the header was loaded or incur high data charges while roaming because those multi megabytes of file attachments that are totally useless on the mobile device have been downloaded as well. On top it's user interface has a number of nice hidden gems that increase productivity and the built in file browser makes things very efficient for quick photo viewing or file attachments. Well, I think you get the message. There's also a version for Android but when I tried some months ago I couldn't get the visual indication working and also for some reason or other the program now and then had issues synchronizing while the phone was in dormant state. Perhaps this is fixed once I make the step from my current Nokia N8 to an Android phone (unless Nokia makes a radical turn around for the good of Meego) but perhaps also not.
As a consequence I gave K9 a try, and I have to say it's a fantastic eMail app for Android which is as configurable as Profimail. And on top, the visible indication for incoming emails and synchronization in the background word flawlessly. In a world where everything has to be simple and non configurable to appeal to a mass audience it's refreshing to see that there are programs out there that can be configured to do just what I want. Great!