Well, of course, in a multi-user system you can send an email to "user". Typically used for system processes to email "root" back in the day, or "postmaster" or other system accounts, as well.

Bang paths host!user or even host1!host2!user were used in the past, and I think they are still supported by most MTAs. They are definitely valid syntax for email addresses. I mean, if you're talking about quotes and parentheses, ...

Canonically postmaster is valid without a domain in SMTP. Traditionally mail messages can be composed to local users without writing domains in their addressses, and the message submission service will add domains where necessary.