If I understand correctly, an alias contains your user name, for example username+alias@protonmail.com. With this it is easy to determine -- and use -- your username@protonmail.com. If you allow users to determine a set prefix (or suffix), to which they add random unique differentiators, outsiders cannot determine the base user account address. My current provider allows 100 of these, and I use them heavily.
Anonymous
If I understand correctly, an alias contains your user name, for example username+alias@protonmail.com. With this it is easy to determine -- and use -- your username@protonmail.com. If you allow users to determine a set prefix (or suffix), to which they add random unique differentiators, outsiders cannot determine the base user account address. My current provider allows 100 of these, and I use them heavily.