For those of us that have more than one email account—and who doesn't these days :), making sure you're sending email from the right account can be something that we're always on guard for. These steps will help make the process a little more fluid.
In this case below, by default when I'm browsing emails in a specific inbox, creating a new message (File -> New Message or Command-N) will create a blank email addressed from the "Mac Zen" account.
Check the Preferences
Navigate to the Mail program's preferences, by going to "Mail -> Preferences" from the menubar at the top, and looking under the "Composing" tab. You will see the behaviour reflected in the setting half way down.
Should you wish to always compose a new message from a particular account, you may set that choice here. It is worth noting however that this setting does not relate to messages that you are replying to. Mail will always reply from the same account that an email has been addressed to.
An Easy Way to Check Which Account You're Sending From
Even though Mail gives you an option to select whcih account an email is being sent from, it's position is in between the subject and main body. Often, by the time we're at the end of an email and ready to send, we're not in the habit of scanning through all of the fields prior to sending. A simple way to make sure that you're always sending from the account you want to, is to take advantage of Mail signatures.
Have a look through the Signatures Tab in Mail's preferences. You have the ability to create signatures for each account that are unique to each account and set a default.
The benefit of this is that you can immediately see when you're composing an email which account you're in by the time you get to the bottom of the email and are making the decision to send. All you have to do at that point is to change the email account rior to sending.