Use only SMTP server without a mailbox

In Mail.app I have several mail addresses and incoming mail to some addresses is already being forwarded to my primary mail address. I don't want to have an inbox for these secondary emails (I've got them on my primary anyway) but I want to be able to send from those addresses. I've tried adding my secondary account as usual, and then disabling it. But then I can't see that address' SMTP server to send from when sending. If I enable it, I can see it, but its mailbox automatically comes back. How can I have no mailbox for a mail account, but still be able to select its SMTP server to send mail from that address?