Using a desktop email client makes this pretty simple- thunderbird, outlook, ect. You can have all emails download into a single folder and reply with whatever email address it was sent to, or you could sent it up so that only one email is used for replies. You can also encrypt the folder that the emails are then stored in- locally.

I personally like this option as I can download and delete all emails off google's servers. My passwords are pretty good, but not great, and if a hacker was able to download ever message I ever sent. :(