„Ever click on a mailto: link, only to scream in frustration as Apple Mail opens yet again? The curse of the webmail user is that there is no way to get around this problem. That is, until now.
Webmailer is an unoriginally named program (actually a preference pane) for Mac OS X that allows you to fix this problem. You can set it to open any URL or run any shell script, complete with the parameters lifted from the mailto link. Webmailer comes with links for several possible webmail sites, including Yahoo! Mail, Gmail, Hotmail, and several webmail programs, such as SquirrelMail or Roundcube. But you can always add your own configuration.
There are two versions of Webmailer, one for Mac OS X v10.3 (Panther) and one for Mac OS X v10.4 (Tiger). The Tiger version includes a feature that allows you to choose your default email handler, be it Webmailer or some other program. Unfortunately, this capability is not available on Panther. Of course, Webmailer is also a Universal Binary, so it will work on both PowerPC and Intel systems.
Hint: Should you ever want to use a configuration other than your default, just hold shift when you open the mailto link.
The Webmailer preference pane, on OS X Tiger“