Apple's Mail app is my email client of choice, but using multiple email accounts on it has brought me to the same frustrating issue: sending email from the wrong account.
Mail often thinks you want to respond from the account that a message want sent to, or from the first account you added to a machine. Fortunately, there's a way to fix this.
1. After opening Mail, click Mail in the Menu bar.
2. Click Preferences.
3. Click Composing.
4. Click "Automatically select best account."
5. Select an address.
Your new emails will now be sent from that address!