Plenty has been written about Shortmail, the “Twitter for e-mail” startup from Baltimore-based 410 Labs, but almost all of it is the usual journalism-by-press-release. Shortmail is one of the few web apps I can remember adopting and finding genuinely useful in the past couple of years, so it deserves a more thorough accounting. Plus, I recently had the chance to talk to Jonathan Julian, one of the developers at 410 labs, and he explained to me some of the logic of Shortmail that I had been missing.

First, a disclaimer: Shortmail is one of those things that you have to try out for a while to really grok, and it’s not for everyone.

Use Case: Public-Facing E-mail

I have no idea how many people make their e-mail address available publicly, so that strangers can send them messages. But I suspect that almost every professional who wants to be reachable does. The proliferation of contact forms on the Web suggests that companies and individuals want to be reachable, but also want a way to filter out the spam.

Shortmail seems almost custom-tailored for this application. In the same way that anyone who is desperate to reach you could send you an @reply on Twitter, Shortmail provides a channel through which anyone can send you a note—but only up to 500 characters in length. It helps that your Shortmail account is by default your Twitter handle @ shormail.com.

As someone who often gets much longer notes from complete strangers, I thought at first this would be a problem. But it turns out that if someone hasn’t made their case in the first 500 characters, I’m not interested anyway. And because Shortmail is a separate channel from my personal email address, it’s an automatic filter for an entire class of mail: First-time connections.

Use Case: Concision

Obviously, replying to people in an environment that demands I use only 500 words is its own time saver. Shortmail provides the perfect excuse for keeping everything brief. It’s the psychology of exchanging messages with an @shortmail.com account, as much as anything, that gives everyone permission to be direct.

Use Case: The Get Things Done Approach to E-mail

I’ve always thought that adherents to the Inbox Zero faith—which demands that you file or reply to every e-mail you get, as soon as you read it—were a special breed. My own inbox is a confusing mess. (Of course, there’s evidence that there’s nothing wrong with neglecting the organization of your e-mail.)

Shortmail’s interface has none of the advanced organizational features of most other Web mail providers, and at first I found it balky. Then I spoke with Jonathan Julian, one of the developers who works on Shortmail, and he said this is intentional.

In Shortmail, messages are viewed one at a time or in threads grouped solely by whom you’re having the conversation with (rather than by particular chains of e-mails). You archive one at a time. It forces you to make simple binary decisions about every e-mail: Respond or banish. The result is that, despite my tendencies, my Shortmail inbox is at zero.

Shortmail apparently works really well with the stripped-down Mac OS email client Sparrow, though I haven’t tried it. There’s also an iOS client. Shortmail supports both IMAP and POP, so you can feed it into any desktop email client.

Use Case: “Open” Conversations

The shortmail team continues to add nifty features to their service, like contact forms for your website, markdown, and what the developers call open conversations. It’s e-mail in public.

In general, Shortmail feels like a niche solution that might take off, or it might not. That’s fine—I’ve been wondering for some time when we’ll see more development firms take the 37Signals route to success rather than the Facebook route. That is, building a service that works for a subset of passionate users, instead of trying to become huge and all consuming and, let’s face it, kind of awful.