Unfortunately, Dropbox only works with Gmail and iCloud. Support for other platforms is coming, though no specifics have been offered by parent company Dropbox.
If you really dig the Mailbox philosophy, then get the Mac desktop app (it's still in beta) for the most fluid email experience. Mailbox is also optimized for the iPhone 6 and 6 Plus.
Most social: Tipbit
The term "contacts" or "address book" harks back to the days when office workers kept all this information on a circular card-holder called a Rolodex.
Now keeping tabs on your contacts' social networks is a vital part of staying connected. Tipbit (free) does a great job at this by pulling in your contacts' Twitter, Facebook, and LinkedIn info--once you grant permission, of course. That way, if you get an email from someone and want to see what they've been tweeting, or need a reminder of their job title, you can just tap on their name and view his or her contact card.
Tipbit also tries to connect a contact with what it thinks is their Twitter account if they don't have one listed in their signiture, but that doesn't always work out--the app showed tweets from Anderson Cooper's Twitter account for one of my contacts named David Cooper.
Tipbit supports email from Gmail, Microsoft Exchange, IMAP, iCloud, Outlook, and IMAP. It is optimized for the iPhone 6 and 6 Plus.
Best customization: Boxer
Boxer (free) has also jumped on the swipe-on-messages bandwagon, though with many different customization opportunities for these actions. It takes a bit of work to set this up, but if you want to work a very particular way and use muscle memory for certain tasks, then you'll like what Boxer has to offer.
It integrates well with features found in other desktop email apps, such as Gmail's labels and sharing files through your Dropbox or Box account. Boxer supports Gmail (it also syncs your labels--a plus for Gmail power users), Yahoo, iCloud, AOL, and Outlook. Microsoft Exchange support only works with the premium version of Boxer, which is $10. It's optimized for the iPhone 6 and 6 Plus.
Best for power users: Accompli
Accompli (free) is the best app of the bunch for power users who get a ton of email and despise constantly moving back and forth to different apps to reference calendar appointments or files.
Accompli's design isn't as polished compared to some of the others, but it compensates with excellent functionality--it includes your Google Drive, Dropbox, Box, or OneDrive contents and calendar events right inside the app.
If you're the business type with multiple meetings and events going on, or if you generally need to plow through a lot of mail during the day, then Accompli should possibly be on the top of your list. It supports Microsoft Exchange, Gmail, Yahoo, iCloud, and any IMAP account; it is optimized for the iPhone 6 and 6 Plus as well.
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