Keitai Mail has English and Japanese localizations, and perhaps German as well (since I think the dev is German). There is a lite version that is free but ad supported, plus a paid version that adds copy and paste from emails, view emoji descriptions (for now only in Japanese, according to the developer), and custom emoji/icon sets. There is also a paid SMS app. Both paid apps are ¥225.
KeitaiMail is currently a beta version, according to the splash screen displayed as the app starts up, which takes longer than average. Being in beta, I can't recommend purchasing, but his app has a lot of potential until google gets its emoji act together, though it is still a bit rough.
Unfortunately, it didn't work at all for me and my Gmail account, but a recent commenter, who turned me on to the app, mentioned that it worked OK for her. To be fair, problems are to be expected because, let's face it, all IMAP clients on Android suck. Period.
The default client is horrible, doesn't work for me at all with my work account, and only supports polling. K-9 Mail, based of the open source default client, is much better and does indeed include push, but still throws up a number of errors for me, tends to copy the same message MANY times into my drafts folder, and will chew through a battery faster than a dog can do the same to one's favorite pair of slippers (even when only using push, or poll at hour intervals) - which is perhaps the reason for the name...?
OK, on to the setup of KeitaiMail.
Here are some of the rough bits that you can probably expect to be cleaned up in later releases.
It doesn't play well with the Simeji keyboard, which is probably the fault of both apps. Pressing the menu button while the Simeji keyboard is displayed is supposed to bring up navigation keys and a softkey for hiding the keyboard. This doesn't work when using this app. Alternatively, pressing the back button on hides the Simeji keyboard, but in this app, it moves you back to the next screen. As a result, most of the following screenshots will have the keyboard displayed because for the life of me I couldn't make it go away. In addition, pressing the back button from the inbox screen restarts the app, rather than taking you to the home screen.
Setting up an account was problematic for me, which is something the developer has addressed on his blog. I first tried using my full email address as username, which resulted in this:
I checked the password and tried again but to the same result, so I deleted the @gmail.com part of my address, as suggested by the developer.
While this didn't return any errors, the mail app continually tried to connect but with no luck. This continued for about 20 minutes until I killed it.
The settings are very sparse, containing only options for how many messages to fetch and how many to display. The other settings are for the custom emoji sets, but this functionality is not available in the lite version. There are no options for setting the client for IDLE (mail push) or polling the inbox, and there is no information about which it is actually doing. I guess it is polling, but at what frequency is unclear. In my experience, setting the frequency too high will kill your battery.
The Japanese localization is very good but it is primarily written in teinei-go, which is something that I don't think I've seen before. Also, I am used to seeing 版 (han) used more often as "version" rather than 板 (han), for example ベータ版 (beta version).
So, I tried to compose a mail. First, since all the carriers use different emoji, you must select the carrier to which you are sending mail. Then enter the recipient's email address. KeitaiMail doesn't provide an option to select from your contacts and didn't register itself as an app for sending mail. This means that when I clicked on an email address while viewing contacts, composing mail with KeitaiMail was not an option and I had to manually type out the address. I was able to compose a mail with emoji, but it failed to send, unfortunately. Finally, I disabled AdFree to see if it had an effect, but was still unable to send or receive mail. Oh, almost forgot, it seems that a description is given for emoji in the lite version. In the below image, when I clicked the sun emoji, 太陽 appeared at the top. Pressing the same emoji again inserted it into the message.
Lots of potential with this, but it needs a bit of work to iron out the kinks.