

Now the next time you go to compose a new email message, either from Mail app or elsewhere, that email account selected will be the new default email address that any new mail is sent from. Look for “Send new messages from:” and pull down the ‘Account of selected mailbox’ dropdown menu to select the email address to use as the new default.Pull down the “Mail” menu and choose “Preferences”.Open Mail app in Mac OS X if you haven’t done so already.You obviously need multiple email accounts setup on the Mac for this to be an option, aside from that this process is the same with any modern version of Mail app for Mac: Changing the Default Email Account in Mail for Mac


Even after setting this you can continue to switch mail addresses easily in compositions. To be clear, this does not disable the other email accounts or disallow their use in Mail app, it simply sets the preferred address to be the default for mail. That includes new mail compositions created from within Mail.app and also elsewhere in Mac OS X, like a mailto link in a web browser or another app. What this is going to do is set the default email account for any new email message composition in the Mail app.
