This patch I wrote adds a command-line option to hide Sylpheed’s main window at startup and quit when done composing. Useful if you don’t like your inbox popping up when you click on a mailto link in a browser and Sylpheed is your default mail app.
Example setting in Preferred Applications of Gnome:
sylpheed --no-main-window --compose %s
Note that this is just a dirty patch and it works with Sylpheed 2.0.4. I sent a proposal to sylpheed developers some time ago, but I don’t know if/when the feature will be implemented properly.