Use only SMTP server without a mailbox
In short, you can't accomplish what you're trying to do. In order to send/reply from a given address, you have to have that account set up and active in Mail.

Without the account being active, the address you wish to use is not available. And setting up an SMTP server entry for the other account(s) will not bypass the primary issue since you would only be using any alternate SMTP server as a relay for the account you're using to send the email - it will not rewrite the sending account address. For example, I have email forwarded from a generic "info@" account to a personal account on the same domain, but I can't reply with or send from the "info@" account address since I don't have that account set up in Mail.