I think Dreamhost discourages created “catch-all” email addresses, but you certainly have “everything” that’s not otherwise specified dumped into one email box.
From there, it would be pretty easy to go into https://mailboxes.yourdomain.com and specify a filter that if the “TO” contains “sub” you want it forwarded to your “newsletter” account.
Whenever I sign up for something, I always use a site-specific email address (ex: email@example.com). It makes it a lot easier to track who is trading and selling names (or had them stolen).
If an address becomes problematic, you can go back to the addresses page on your panel and have all mail to that address get automatically deleted.