How to organize/receive emails on one account while sending from another
If you want all email send from one account (i.e. gmail) you can set that up in Mail preferences.

Open Mail Preferences then select Composing:



Now select your gmail in the "send new messages from:"

Obviously in Mail you will need Yahoo and Gmail accounts.

Alternatively you can use the "Reply to" in Mail app.

For that Start a new mail and go View - "Reply to Address Field" or simply use shortcut alt+cmd+R. Then enter your reply to address. Now when you send new emails or you reply to emails it will use the selected email.



The third option: if you are really serious with that, you can use the Terminal app located in your Utility folder.

Type "defaults write com.apple.mail UserHeaders '{"Reply-To" = "reply-to@address"; }'" (excluding the outermost quotation marks) and replace reply-to@address with the address you want to be used for replies.

Read more here