Gmail Sender Guard

Gmail Sender Guard checks the From address before you send. On a reply, it looks at which of your addresses the original message was sent to. If you were only Cc'd, it uses that address. If your From is different, it asks whether to switch. On a new email, it looks at which address you usually use for those recipients. When that's clear, it offers to switch. When you've used more than one, you pick an address and can save it as the default for that person.