Hi,

You can already do this in SmarterMail. Just go to domain.com → Users  → Aliases, then create a new alias with the user’s specific email address. Make sure the Show in GAL option is turned off so the alias remains hidden. You can also enable the option to allow send from alias, which lets the user send mail using that address as their From address. This way, the alias will function without being visible in the Global Address List.

Thanks,