Mail alias overridden by nonexistent user forwarding

[Claude@]

I would like to divert spam from my catchall address. I know that a handful of addresses commonly receive spam, so have set up aliases for them, to a spam@domain-name mail account. That spam mail account has no mailbox (i.e. messages get discarded).

I also have all mail to nonexistent users forwarded to my default mailbox, as I like creating new email addresses for different websites on the fly.

I would presume that if an alias exists (e.g. alias1@domain-name and alias2@domain-name belong to normal@domain-name), mail to that alias1@ would appear in the normal@ mailbox.
However, I'm finding messages to alias1@ appearing in my catchall mailbox, which gets mail to nonexistent users.

Is this a bug? I would have thought the desired behaviour is that once an alias is created, it's no longer a nonexistent user...

any advice appreciated. I don't want to have to create a mailbox for each and every address that gets spam, on every domain (simply creating several aliases on one spam mailbox is much easier).

thanks,
Claude