On the scale of global problems desperately in search of a solution, email overload may not be as dire as malaria or clean water shortage. But in the first world, it’s a full blown crisis. And SaneBox has taken it on.

The issue disproportionately affects those who are considered important or influential but is nonetheless getting worse by the minute, meaning there is a lot of money and attention being paid to solutions. By 2008 it had already gotten so bad that Mike Arrington issued one of the first public email overload challenges to entrepreneurs. MG Siegler very publicly quit email for one month last summer and has since been on a crusade against his inbox. Paul Graham called the solving of our collective email woes a frighteningly ambitious startup idea and dared anyone brave enough to tackle the issue.

Of the non-nuclear options, those which don’t involve quitting or replacing email entirely, SaneBox looks like one of the best.

The self-funded company offers priority filtering, newsletter subscription management, and follow up management. Each of these features is fully customizable by users, but in many cases it’s not required. The company also disclosed that it will soon offer automated email attachment management and storage through a Dropbox integration.

The results are so good that users have been happy to pay for the service, a rarity in today’s online world, and once they start, they’re extremely unlikely to stop. The company released some subscriber statistics for the first time today, which illustrate this point.

Nearly one in three people who sign up for a free two-week trial become paying users. This number was nearly one in two over the last week, although the company doesn’t expect that to become a trend. More tellingly, less than one percent of users unsubscribe each month, making it among the “stickiest” paid services around. These strong figures have enabled the company to grow to this point without raising any outside capital, and it’s expecting to reach breakeven later this year.

This is pretty impressive for a company that charges for its product and competes directly with the solutions offered for free by others, including Gmail, the email platform of choice for the majority of SaneBox subscribers. In fact, Gmail’s competing Priority Inbox launched without warning, while SaneBox was in private beta. Gmail actually took a large number of SaneBox’s users, though most were dissatisfied and ended up coming back shortly thereafter eager to resume paying for their preferred solution.

Another free would-be challenger and arguably its biggest third party competitor is OtherInbox. While it could be argued that the service is inferior (and it is), all users really need to know is that it was recently acquired by Return Path, an “email certification and reputation monitoring company.” Return Path counts email marketers, aka spammers, as its main clients. That’s about the last company anyone needs poking around in their inbox.

Although still relatively small in size, SaneBox is growing quickly, having tripled users since January with growth continuing to accelerate. The company powers its prioritization algorithms in part through social network integration, contributing partially to the viral growth of the service.

SaneBox works by moving unimportant emails out of the inbox in a variety of ways, either for later review or to never be heard from again at the user’s choice. The company’s SaneLater folder (or label in Gmail), intelligently sorts all mass-marketing mail and other non-essential messages. The company claims that SaneLater reduces the number of messages in a typical inbox by about 58 percent. Custom folders such as “Family” and “Receipts” can be created to function similarly.

In addition to prioritizing incoming mail, SaneBox simplifies unsubscribing from bulk mailing lists and offers smart follow up reminders. SaneBox takes a rather diplomatic course in “unsubscribing,” such that it doesn’t actually remove users from distribution lists. Users simply drag undesired emails into the specified folder and the service creates smart filters that automatically delete inbound emails before they ever hit the inbox. This is awfully generous to marketers concerned with distribution statistics, but it does avoid companies selling contact information of unsubscribes.

SaneBox also improves on the email follow up abilities of stand-alone solutions such as Nudgemail and followup.cc. Through its SaneRemindMe feature, users simply “cc” or “bcc” a time parameter, like “5h, tomorrow, Wednesday” or “April 27,” and SaneBox sends a reminder email at that specified time. Once the recipient replies to the original email, the reminder is canceled instantly.

The company’s customers are both individual consumers and those in enterprise. An obvious concern for either is privacy and security. The company does not read or download user emails to its servers. Its system relies on analyzing email headers only. As a paid solution, it does not sell user data or serve ads based on the limited information it does see. By comparison, Gmail sells targeted ads based on the body content of user emails.

Many power users will not be satisfied until the email becomes less of a to-do list and more of an obligation-free broadcast system along the lines of Twitter. Until that day comes, SaneBox seems to offer a pretty workable solution to email overload.