I use BlueMail myself. I would normally take the open source option, but K9 didn't work with so many servers. In desperation I went back to GMail, but it didn't handle rfc822 attachments. Then Microsoft Outlook, but it just didn't work period on many occasions. I tried a whole pile of others in rapid succession, which is how I came across BlueMail.

It works with every service, handles the broadest range of attachments, is perfectly reliable, has no ads and has the best UI of any of them. I don't know why it exists, but I'm glad it does.

Still, it not being open source hurts. I wish Thunderbird worked on Android.