While technically possible to configure a DNS zone for a top-level domain (TLD) like .net or .com to accept MX records, this violates IETF standards and faces practical barriers:

; Example DNS zone file for TLD (not recommended)
net.    IN  SOA   ns1.iana-servers.net. hostmaster.icann.org. (
            2024022001 ; serial
            7200       ; refresh
            3600       ; retry
            1209600    ; expire
            3600       ; minimum
        )
net.    IN  MX  10 mail.net.
mail.net.  IN  A  192.0.2.1

Three critical issues prevent functional implementation:

• Registry Restrictions: ICANN-accredited registrars prohibit direct registration of TLDs
• Mail Server Rejection: Most MTAs filter such addresses via SPF/DKIM checks
• RFC Violation: RFC 5322 requires domain parts to be FQDNs with subdomains

Hypothetical solutions with their flaws:

# Python email validation snippet
import re
from django.core.exceptions import ValidationError

def validate_email(value):
    pattern = r'^[a-zA-Z0-9_.+-]+@([a-zA-Z0-9-]+\.)+[a-zA-Z0-9-]{2,}$'
    if not re.match(pattern, value):
        raise ValidationError("Invalid email format")
    
    domain_part = value.split('@')[1]
    if len(domain_part.split('.')) < 2:
        raise ValidationError("TLD-only domains not permitted")

For those needing ultra-short domains:

• Use ccTLDs with short SLDs (user@x.y in .co.nz or similar)
• Register single-character domains where available
• Configure domain aliases through your mail provider

The last functional TLD email was postmaster@ai (Anguilla's ccTLD) in 1995 before ICANN policies changed. Modern implementations would break:

// Node.js email validation
const { isEmail } = require('validator');

console.log(isEmail('test@com')); // Returns false
console.log(isEmail('test@example.com')); // Returns true

In standard email addressing (RFC 5322), the format requires local-part@domain. The domain portion typically consists of multiple levels:

example.com = 
├── com (TLD)
└── example (second-level domain)

Technically speaking, the DNS system doesn't prevent using just a TLD as the domain part. The real constraints come from:

• Domain registry policies
• Email server configurations
• SMTP protocol implementations

While the syntax would technically permit user@net, practical implementation faces multiple hurdles:

// Example DNS query for TLD MX records (would typically fail)
dig MX net +short

Most TLD registries prohibit direct registration under the TLD. The few exceptions are:

For developers implementing email validation, you should account for this theoretical possibility:

// JavaScript validation that permits TLD-only domains
const emailRegex = /^[a-zA-Z0-9.!#$%&'*+/=?^_{|}~-]+@([a-zA-Z0-9-]+\.)*[a-zA-Z0-9-]+$/;
// This would technically allow 'user@net' but see caveats below

Even if you could register a TLD as a domain, email servers face delivery challenges:

# Postfix configuration snippet showing typical domain requirements
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
# Most servers expect at least a second-level domain

The hierarchical nature of DNS means most mail servers expect to find MX records at the second level or below.

In the early days of the Internet (pre-1984), some email systems actually used formats similar to this. The @host format was common on ARPANET before the DNS system we know today was established.

Modern exceptions include:

• .onion addresses for Tor network
• Special-use domains like .test
• Internal network implementations

When building email-related functionality:

1. Validation regex should technically permit TLD-only domains
2. But anticipate most real-world systems will reject them
3. Consider adding a warning for unconventional formats

// Python example showing validation with warning
import re
from django.core.exceptions import ValidationError

def validate_email(value):
    pattern = r'^\w+@([\w-]+\.)*[\w-]+$'
    if not re.match(pattern, value):
        raise ValidationError("Invalid email format")
    if '.' not in value.split('@')[1] and len(value.split('@')[1]) < 4:
        warnings.warn("Unconventional domain format may cause delivery issues")