When implementing DNS CNAME records for domain aliasing, many administrators encounter unexpected behavior with MX records. The technical reality is that RFC 2181 explicitly states that no other resource record types (including MX) should coexist with CNAME records at the same node.

DNS resolvers follow a specific processing order when querying records. A CNAME at the apex (naked domain) effectively hijacks all other record lookups for that domain. This occurs because:

1. The resolver first checks for a CNAME at the queried name
2. If found, it follows the CNAME chain
3. Original query type (MX in this case) becomes irrelevant

Here's a problematic zone file example:

; BAD configuration - will break MX lookups
example.com.    IN CNAME   client.dns.provider.com
www             IN CNAME   client.dns.provider.com
example.com.    IN MX 10   mail.example.com
mail            IN A       192.0.2.1

The correct approach would be:

; WORKING configuration - uses ALIAS/ANAME or direct A records
example.com.    IN ALIAS   dns.provider.com  ; Non-RFC solution
www             IN CNAME   client.dns.provider.com
@              IN MX 10   mail.example.com
mail           IN A       192.0.2.1

Many DNS providers now offer special record types to solve this:

• ALIAS (DNSimple, AWS Route53 alias)
• ANAME (DNS Made Easy)
• Flattened CNAMEs (Cloudflare)

Example using Route53:

{
  "Changes": [{
    "Action": "CREATE",
    "ResourceRecordSet": {
      "Name": "example.com",
      "Type": "A",
      "AliasTarget": {
        "HostedZoneId": "Z2ABCDEFGHIJKL",
        "DNSName": "d-1234567890.cloudfront.net",
        "EvaluateTargetHealth": false
      }
    }
  }]
}

Always test your DNS configuration with:

dig example.com MX +short
dig example.com CNAME +short
nslookup -query=mx example.com

Remember that DNS changes may take time to propagate globally due to TTL settings.

When implementing CNAME records at a domain's root (apex), you're essentially telling DNS: "This domain is just an alias for another domain." The critical observation here is that RFC 1034 section 3.6.2 explicitly states that CNAME records must be the only record type for a given domain name. This means all other record types (MX, TXT, NS, etc.) become inaccessible when a CNAME exists for the same name.

Let's examine the DNS query behavior using dig:

# Before CNAME implementation
dig mx example.com +short
10 mail.example.com

# After implementing CNAME
dig mx example.com +short
client.dns.yourserver.com

The MX lookup returns the CNAME target instead of the actual MX record because the CNAME takes precedence at the DNS resolution level.

Option 1: ALIAS/ANAME Records

Many DNS providers offer virtual alias records that behave like CNAMEs but don't conflict with other record types:

; Cloudflare example
example.com. IN ALIAS client.dns.yourserver.com
mail.example.com. IN A 192.0.2.1

Option 2: DNS Provider APIs

For automated management during server migrations:

# Python example using Cloudflare API
import cloudflare

def update_dns(domain, new_ip):
    cf = cloudflare.CloudFlare()
    zone_id = cf.zones.get(params={'name': domain})[0]['id']
    records = cf.zones.dns_records.get(zone_id)
    
    for record in records:
        if record['type'] == 'A' and record['name'].startswith('client-'):
            cf.zones.dns_records.put(
                zone_id,
                record['id'],
                data={
                    'type': 'A',
                    'name': record['name'],
                    'content': new_ip
                }
            )

A common architecture pattern that avoids this issue:

; DNS Zone File
client1.example.net. IN A 203.0.113.45
mail.example.net.    IN A 203.0.113.46

; Client domain
client.com.          IN MX 10 mail.example.net.
www.client.com.      IN CNAME client1.example.net.

Always validate your DNS configuration with:

dig mx example.com +trace
host -t soa example.com
nslookup -type=mx example.com