There are some ways, however, to fiddle with Mail to make it more difficult to check in a moment of weakness. First, go to Preferences and, under General, set "Check for new mail" to "Every hour." Next, go to the View menu and select "Customize Toolbar..." Get rid of the "Check mail" icon, and replace it with whatever you want (I chose "Smaller/Bigger," for those folks who send micro-sized text).
It's still possible, of course, to check mail by hitting a quick command-key sequence. To fix that, download CocoaSuite, a collection of utilities that lets you modify, among other things, menu behavior. After it's been installed, launch Mail, go to Manage Menu Shortcuts (under Mail / Cocoa Suite), select "Mail / About Mail" from the drop-down menu, and enter the command sequence shift-command-N. Click Save, quit, restart, and you should see that "Check All New Mail" no longer has a keyboard equivalent.
Sure, you can still drag down the menu and select "Check All New Mail" manually, but we've at least raised the bar a bit, to the point that checking mail is more than just a quick click. I find that I'm now content to wait until the hour has passed to see whether there's any mail. And if there were an option to wait longer, I'd probably try that, too.
And yes, to anticipate one likely response, one could just quit Mail entirely. But starting Mail is even easier than checking it....

