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DKIM signing is optional for MailChannels to function on cPanel, but it adds an authentication layer that improves deliverability. MailChannels does not modify the parts of messages you sign with DKIM, so existing signatures remain valid. This guide assumes you have already completed the base cPanel/WHM setup.

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Add DKIM to the Exim transport

In WHM, navigate to Service Configuration > Exim Configuration Manager > Advanced Editor and locate the mailchannels_smtp transport block in the TRANSPORTSTART section.

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Exim 4.93 and earlier

Append the following lines to the end of the mailchannels_smtp transport block: 
dkim_domain = $sender_address_domain
dkim_selector = default
dkim_canon = relaxed
dkim_private_key = "/var/cpanel/domain_keys/private/${dkim_domain}"
The completed transport block will look like this:
Section: TRANSPORTSTART
mailchannels_smtp:
  driver = smtp
  hosts_require_auth = *
  tls_tempfail_tryclear = true
  headers_add = X-AuthUser: ${if match {$authenticated_id}{.*@.*}\
    {$authenticated_id} {${if match {$authenticated_id}{.+}\
    {$authenticated_id@$primary_hostname}{$authenticated_id}}}}
  dkim_domain = $sender_address_domain
  dkim_selector = default
  dkim_canon = relaxed
  dkim_private_key = "/var/cpanel/domain_keys/private/${dkim_domain}"

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Exim 4.94 and later

Exim 4.94 introduced tainted filename restrictions that affect how ${dkim_domain} is expanded. Use the get_dkim_domain Perl function instead:
Section: TRANSPORTSTART
mailchannels_smtp:
  driver = smtp
  hosts_require_auth = *
  tls_tempfail_tryclear = true
  headers_add = X-AuthUser: ${if match {$authenticated_id}{.*@.*}\
    {$authenticated_id} {${if match {$authenticated_id}{.+}\
    {$authenticated_id@$primary_hostname}{$authenticated_id}}}}
  dkim_domain = ${perl{get_dkim_domain}}
  dkim_selector = default
  dkim_canon = relaxed
  dkim_private_key = "/var/cpanel/domain_keys/private/${dkim_domain}"

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Apply the changes

After saving the Exim configuration, restart Exim to activate the new settings: 
/scripts/restartsrv_exim
Send test messages to confirm that outbound mail is being signed and that signatures validate correctly at the receiving end.