zimbraAllowAnyFromAddress allows people to specify a persona that has any address as the from address. This has nothing at all to do with aliases.
An alias is an alternative address that the user can also recieve mail for. To specify an alias of
account@otherdomain.tld your Zimbra server would need to be told that it can handle addresses ending in otherdomain.tld. (Aliases also allow the user to specify that address as a from address, regardless of the setting of zimbraAllowAnyFromAddress).