I've got a nice little mail server setup (running Ubuntu Server 16.04) that works a treat. It uses postfix as SMTP, which uses dovecot (IMAP) as an authentication source for virtual mailbox users.
It's been working fine, but now I have a second server in another location (with a dynamic IP address), which I'd like to be able to send mail through my main mail server. I've seen a guide on setting it up in authenticated-relay mode, and am part-way through the process, but then I realised that I'd need a send-only email account for it to authenticate against, as I don't want it to be able to receive mail and fill up the server's hard drive (I won't check the inbox!).
How can I create a new email account in my virtual users setup that doesn't have a mailbox, but can still connect via SMTP and send emails?