The team behind Mailbox wrote that the app -- which delivers Gmail messages to the iPhone or iPod Touch -- will remain, but it needs a company like Dropbox to help it grow. It wants to add more e-mail providers and build apps for more devices, but the 14-person company doesn't have the staff to do those things.
"To be clear, Mailbox is not going away," the post reads. "The product needs to grow fast, and we believe that joining Dropbox is the best way to make that happen. Plus, imagine what cool things you could do if your Mailbox was connected to your Dropbox..."
A third-party e-mail client, Mailbox launched last month and quickly built up a long waiting list of customers. The company's service capacity has grown 2,000 times since it launched and it's delivering more than 60 million e-mails a day, according to Mailbox.
Dropbox is rumored to have paid as much as $100 million in cash and stock for the startup, according to unnamed sources in a TechCrunch report.
The companies aren't saying. What Dropbox did say was that Mailbox solves the common problem of e-mail overload and simplifies a task, and that's in line with the storage company's mission.
"Like many of you, when we discovered Mailbox we fell in love -- it was simple, delightful, and beautifully engineered," reads the Dropbox post. "Many have promised to help us with our overflowing in-boxes, but the Mailbox team actually delivered."
Update, 5:45 p.m. PT: Adds information on rumored purchase price.