If you think some of the stock email providers are missing features, K-9 Mail is a good alternative. K-9 Mail offers some much-needed features like searching multiple in-boxes, the ability to swipe for previous/next emails and useful keyboard shortcuts.
For me, the setup of several mailboxes took too long to complete. I used the standard IMAP for Gmail, POP3 for a mailbox setup specifically for this test and an Exchange mailbox. After a few tries, I was able to set up the mailboxes, but the amount of time it took almost made me abandon the app.
When the app finally worked, I was easily able to read and reply to messages. I loved that I was able to use keyboard shortcuts (I tested this on my old G1), and was able to swipe through the emails.
One of the added features that's nice is the ability to search through multiple in-boxes. If, like me, you have multiple in-boxes and don't know where an email was received, you simply search for it, and the app will retrieve the email regardless of which inbox it came from.
Some areas for improvement for this app are the UI (needs some polishing), and the app was somewhat sluggish. After reading other reviews and comments, this seems to happen when there's a lot of traffic in your mailboxes.
If you're looking for more than just the stock mail application, K-9 Mail is a great alternative, with some helpful added features.