Pat Dryburgh, writing for The Brooks Review, has a great idea:
Dropbox gives its users 2 GB of free storage. That is certainly more than most email users today need to archive all of their email attachments. Combining that 2 GB of file storage capacity with a full-fledged cloud-based email service would attract a lot of casual users. Dropbox has established its own referral/invitation strategy, which benefits all three parties involved in the transaction. Implementing the same strategy to a product as compelling as ad-free email could surely drive their growth over the next few years, if not longer.
And then, of course, we come to the most exciting part. Dropbox would charge for it. In my mind, it would make sense to simply add the email service to their existing pricing tiers in an effort to improve their overall product. Adding lower tiers, more consumer-friendly tiers, could convert many free-mail users into paying email customers.
As a devout Dropbox user, I'd for sure pay for email syncing/archiving. The only barrier to me using Mailbox now is that it's Gmail-only, and I don't use Gmail. Should they add a thing like the one Pat suggests or add support for other IMAP accounts (in my case, iCloud), I'll absolutely check it out.