I haven't found this specific case in an RFC, but checked Go's net/mail
library behavior and it just considers it broken:

$ cat mail.go
package main
import "fmt"
import "net/mail"
func main() {
    fmt.Println((&mail.AddressParser{}).Parse("a@example.com"))
    fmt.Println((&mail.AddressParser{}).Parse("a@malicious.org@example.com
"))
}

$ go run mail.go
<a@example.com> <nil>
<nil> mail: expected single address, got "@example.com"

That would fix the security issue but not the whole ticket.