How to get Mac Mail to show different Gmail addresses in the reply to field? I have a primary gmail account: me@gmail
My employer set up email that forwards to gmail as a different identity: me@mtk.org

Me@gmail downloads to my Mail inbox on Mac. I can reply to messages sent to thus account from my Mac. 

Mail sent to @mtk.org Also downloads to the mail inbox on the Mac   BUT I CANNOT reply to these messages from the mtk email - it does not show up as an option in to select in the From drop down menu in Mail.  It would not be appropriate for me to reply to these messages from my personal Gmail address. Work email needs to be responded to from the work address   

How do I get the MTK email address to show up in the drop-down menu on the Mac so that I can reply using that address?
I use Gmail for Work along with Mac Mail and how I got around this was by opening:


Preferences > Accounts
Then in the Email Address field I put a comma after the address that was there and then added the other address I wanted to be able to reply from. For me I added 4 different ones just by separating with a space and comma like this:


me@gmail.com, me@mtk.org, sales@mtk.org

Now, when I compose an email, there is a a drop down menu that allows me to choose which address I want to send my message from. The only caveat here, is that all of my addresses end in the same business name and it sounds like yours don't. I don't know how your boss has gmail for work set up, but this is easy to change and worth a try! If it doesn't work you can just go back to how the account was setup before, no harm, no foul.