Thanks I get it now
For anyone looking for similar info:
Forward-only does not go through DH server at all, therefore DH quota will not count. If you set a fully hosted email to forward mails to another server (in my case yahoo), regardless of sender address, it will still use DH quota.
Easiest way to confirm this: in your control panel, go to email. For full hosted addresses you will see a link called mailbox manager. If you can click that link and log in, that address will use DH quota. If not, then it is forward-only/garbage, and will not use DH quota.
A DH tech support also answered my question about this, so here is her summary:
The simple answer is that SMTP quotas apply to full mail accounts that
send mail from DreamHost’s servers. If firstname.lastname@example.org was
a full mail account with an inbox and a login & password, all the emails
that it forwards to yahoo would be counted against the quota limits.
Mail sent from Yahoo’s website, if it is done using an SMTP connection to
login to the DreamHost mail account, would also count against the quota.
BUT, for you and your setup, because you have
email@example.com forward-only without a login, there is no
SMTP quota. If you have any other questions or issues, please let me
know, I’m always happy to help!