We have two internet domain names. If I set a default internet domain and tick the "recipients are known exclusively by this internet domain name" option, Can I change this lower down for individual users to pick the other internet domain name? Will external email still be sent/delivered?

Some of our users have both internet domain names, but most only have the default. At the moment both email domain names are available to all users even though the internet domain name is set to our default (ticked).

Thanks