When implementing DKIM across multiple sending sources (Exchange, cPanel, third-party services), you're essentially dealing with a public key infrastructure (PKI) where each sender needs proper key management. The core issue isn't just cryptographic signing - it's maintaining consistent authentication when emails originate from different infrastructure components.

For environments with mixed sending platforms, consider this key management approach:

# Example DNS TXT record structure for multiple selectors
selector1._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC..."
selector2._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC..."

For services lacking native DKIM support, implement these workarounds:

1. Request the vendor provide SMTP headers for manual signing
2. Set up a proxy MTA that adds DKIM signatures post-send
3. Use services like AWS SES that can inject DKIM headers

For Exchange environments, you'll need transport rules:

# PowerShell example for Exchange DKIM
New-TransportRule -Name "Apply DKIM" -SenderDomainIs "example.com" 
-ApplyHtmlDisclaimerLocation "Append" -HeaderMatchesMessageHeader "DKIM-Signature" 
-HeaderMatchesPatterns "v=1; a=rsa-sha256; d=example.com; s=selector1"

cPanel's automated DKIM setup often needs manual refinement:

# /etc/exim.conf modifications for custom selectors
dkim_selector = selector2
dkim_domain = example.com
dkim_private_key = /etc/domainkeys/example.com/selector2.private

When configuring services like Mailchimp or SendGrid:

/* API example for service configuration */
{
  "domain": "example.com",
  "dkim": {
    "selector": "selector3",
    "public_key": "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA..."
  }
}

To avoid sudden deliverability drops:

• Phase in DKIM by service tier (transactional first)
• Monitor authentication results using DMARC reports
• Maintain parallel old/new key pairs during transition

Essential diagnostic commands:

# Check DNS propagation
dig +short selector1._domainkey.example.com TXT

# Verify email signatures
openssl dgst -sha256 -verify publickey.pem -signature signature.txt message.txt

DKIM (DomainKeys Identified Mail) operates through cryptographic signing where the sending server signs messages with a private key and receivers verify using the public key published in your DNS. In multi-server scenarios, each sending source requires its own DKIM key pair and DNS record.

For your specific setup with Exchange, cPanel and third-party services, you'll need:

; DNS zone example
mail01._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GC..."
mail02._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GC..."
service1._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GC..."

For services without DKIM support:

1. Route emails through your DKIM-enabled MTA first
2. Use Sender Policy Framework (SPF) as fallback
3. Consider dedicated subdomains for unsigned services

Exchange Server DKIM setup:

# PowerShell for Exchange transport rule
New-TransportRule -Name "DKIM Signing" -Enabled $true
-SenderDomainIs example.com -ApplyHtmlDisclaimer "DKIM-Signature"

cPanel/WHM DKIM setup:

# SSH access required
/usr/local/cpanel/bin/dkim_keys_install --all
/usr/local/cpanel/bin/dkim_keys_enable --all

Most modern mail servers:

• Check for DKIM when available but don't mandate it
• Use DMARC policies to determine handling of unsigned mail
• Apply cumulative scoring (SPF + DKIM + other factors)

Essential verification commands:

# Check DNS propagation
dig +short txt mail01._domainkey.example.com

# Verify message signatures
openssl dgst -sha256 -verify pubkey.pem -signature sig.txt message.txt