You've already got individual ringtones set up for your boss, your spouse, and other important people. MailTones brings that same concept to e-mail, playing custom sounds when you receive messages from those certain special senders.
Using the $2.99 app [iTunes link] requires two things: That your iPhone is configured for push e-mail (fetched automatically, not manually) and that your e-mail server has the option of forwarding copies of new messages to another address.
Gmail, for example, makes this a snap, as does MobileMe. As for other services and servers, well, you'll have to find out if forwarding is an option. If not, this isn't the app for you.
With that single step completed, however, all that remains is to configure MailTones' alerts, which offer three options.
Friend Matching works like call-based ringtones: You get an alert when you receive mail from a specific person. (However, you can't just enter someone's e-mail address; you have to choose someone already in your address book. Minor gripe.)
Subject Matching sounds an alert when the subject line contains one or more keywords. And Domain Matching targets mail from specific domains (like, say, the office).
For each match you set up, you can choose from 25 available tones, which range from short, simple "dingtones" (my word) to boisterous cowbells, ship horns, and fire sirens.
Suffice it to say, most people get a lot more e-mail than they do phone calls, and it's easy to experience alert overload. Consequently, you'll almost certainly want to set the New Mail tone to "Ignore" so you don't get interrupted every 10 seconds.
At the same time, turn on Show Alerts, which provides a text message-like pop-up (with the sender's name and subject line) along with the audio alert. That way you'll know if the message is important enough to warrant a full-on visit to Mail.
MailTones is an idea whose time has come. If you're tired of pulling out your iPhone every five minutes to see if that new message is something important, now you've got an audio-cue alternative. Indeed, for anyone inundated by e-mail, MailTones is $2.99 well spent.