Set your From address or your Reply To address in Mail on Mac
If you have multiple email addresses, you can choose an address to send your messages from and another address to receive replies.
Set your From email address
If you set up email aliases, use several email accounts or use unique, random addresses created with Hide My Email, you can choose which address to use when you send your messages.
In the Mail app
on your Mac, move the pointer over the From field in your message.Click the pop-up menu that appears, then choose an email address.
If you want to use the same From address for all your messages, choose Mail > Settings, click Composing, click the “Send new messages from” pop-up menu, then choose an email account. Or choose instead to have Mail automatically select the best address, based on the email address of the first recipient in your message, as well as the currently selected Mailbox and message.
Set your Reply To email address
You can specify the address where you want to receive replies to your message.
In the Mail app
on your Mac, click the Header Fields button 
in the toolbar of the message window.Choose Reply-To Address Field, then enter the address where you want to receive replies to your message.