I’m currently running Apple Mail with the unified inbox option, so I can connect to my old company email address as well as my Code for the People address and see everything in one place. There’s settings in Mail to allow you to set the default sending address, and when you reply to a mail it should pick the correct account. Trouble was, I kept noticing that my mail was going out under the new account when I was sure I’d specified the new account to be the sending address. Infuriating and embarrassing.
After a few weeks of getting increasingly annoyed with Apple Mail, the problem, as usual, turned out to be elsewhere. Both the old and the new email addresses are part of separate Google Apps setups. When I configured my mail accounts in Apple Mail, I hadn’t been careful when choosing my Outgoing Mail server… both of them were set to the old email account, and Google was silently switching my sender address inside the outgoing mail server.
To fix this, go to: Preferences > Accounts and then pick an account. In the drop down for Outgoing Mail Server (SMTP) choose Edit SMTP Server List…. Make sure that all your SMTP servers are named clearly and have the right credentials in them, you should have one SMTP for each of your Google Apps accounts. Then Click OK to close this dialog. You are now back on the Accounts preferences screen, now make sure the right outgoing server is selected in each of your accounts.