If you want to be able to reply with a specific FROM email address, you need to set up this email account in Apple Mail.
When the new email account has been set up, you'll under the new email account's 'Outgoing Mail Server (SMTP)' setting have the server to use when replying - and by having this new account, you'll be able to choose it from the drop-down as the reply FROM address.
When you have set up the new mail account for @mtk.org, you'll have set up (configured) the Outgoing Mail Server too - and this Mail Server could be using your current GMail Server identity setup, which will work the same as 'kristcarmen' suggested in his answer, or it could be the correct Outgoing Mail Server used with @mtk.org, if the last, then all @mtk.org mails will be replyed 'the correct way'.
Needed server identity for your @mtk.org Mail Server: https://support.apple.com/en-us/HT203995 - if you look at your current GMail account set up, you can see what you need - so you need to get the same kind of server identity informations for setting up your @mtk.org account.
Incoming mail server settings:
- Account Type: Choose IMAP*, POP, Exchange IMAP, or Exchange EWS1, as directed by your email provider. If you're using a Microsoft Exchange account, see the Exchange setup instructions for Mac or iPhone, iPad, and iPod touch.
- Incoming Mail Server (host name): The host name of the incoming mail server, such as mail.example.com.
- User Name: Your user name for this account, such as appleseed. Some email providers want your full email address as your user name.
- Password: The email password you use to sign in to your account.
- Port: The port number used by the incoming mail server. Common port numbers for incoming mail are 143 and 993 for IMAP accounts, and 110 and 995 for POP accounts.
- Authentication: Choose Password, MD5, NTLM, Kerberos, or None, as directed by your email provider.
- Use SSL? Does the incoming mail server support SSL (Secure Sockets Layer) or TLS (Transport Layer Security) encryption?
Outgoing mail server (SMTP) settings:
- Outgoing mail server (SMTP): The host name of the outgoing SMTP (Simple Mail Transfer Protocol) server, such as smtp.example.com.
- Port: The port number used by the outgoing mail server. Common port numbers for outgoing mail are 25, 465, and 587.
- Authentication: Choose Password, MD5, NTLM, Kerberos, or None, as directed by your email provider. If None, you may need the additional settings below to send email when you're on a different network, such as from a Wi-Fi hotspot or Internet cafe.
- Use SSL? Does the outgoing mail server support SSL or TLS encryption?