This was something that bugged me for quite some time before I stumbled upon that fact that it is painfully simple.  I mean, it is a Mac.

Whenever a link from a website or program triggered my default mail program it would open a new message using an email account that I don't use often.  Not a big deal, but a few seconds here and a few seconds there add up throughout your day.

Mail.app simply uses the first mail account as the default account.  Changing the default account is as simple as dragging your preferred account to the top.  This can be done in the main app sidebar or from the preferences.