Could "[email protected]" be a valid email address?

Solution 1:

No, this would not be a valid email address. The host part of an email address (@example.com), can be an IP address instead of a domain name, but the IP must be enclosed in brackets, for example:

username@[127.0.0.1]

See RFC 5321 Section 4.1.3:

Sometimes a host is not known to the domain name system and communication (and, in particular, communication to report and repair the error) is blocked. To bypass this barrier, a special literal form of the address is allowed as an alternative to a domain name. For IPv4 addresses, this form uses four small decimal integers separated by dots and enclosed by brackets such as [123.255.37.2], which indicates an (IPv4) Internet Address in sequence-of-octets form.

That said, in practice an email address like using an IP like this will trigger spam prevention measures all over, so it wouldn't be very practical. But it is technically a valid email address.