Let’s talk about how you can reply to email messages with the same email address the message was sent to. This is helpful if you have set up multiple email accounts to forward to your Main GMAIL account.
Log into your Main GMAIL Account.
Go to Settings, Account and about half way down the page, look for Send Mail As: You need to select one email address as your default, this is what will be used to send messages when you compose a new message. To reply as other email addresses you own (that you have set up to forward), select Add another email address you own. Please read about treating email as alias here to make your choice. Most of mine are set up as an alias, but you should read about the difference and make your choice.
Follow the steps you see in the pop up box. You can set up to use the email servers of your forwarded account if you would like, be sure and have the smtp server name, username, and password handy if you choose to send email that way. You will have to enter a confirmation number just like you did when forwarding email.
You can set up multiple email addresses to reply as the same email account it was sent to. See my various email addresses feeding into my main GMAIL Account here:
As you can see above, there is an option under When replying to a message to choose between always sending from your default email account (this is helpful if you want to change your email address) or if you want to reply to the same edress the message was sent to.
You may also want to set up multiple email signatures for each account. See this post: GMAIL | Using Signatures.