Problem

Joomla does not provide configuration fields for setting DKIM parameters when using the PHP Mail or Sendmail sending methods. Although both methods use the underlying PHPMailer class (which supports DKIM), there's currently no way to set required fields like DKIM selector, domain, or private key via the Global Configuration.

As a result, outgoing emails do not include DKIM headers, which may lead to email delivery issues and DKIM test failures.

Email providers like Gmail, Yahoo, and Outlook are enforcing stricter policies for unauthenticated emails. Studies and real-world data show that emails without proper DKIM and SPF setup can experience up to 30–50% higher chance of being flagged as spam or rejected entirely, especially when sending from custom domains. This is expected to worsen as spam filtering policies continue to tighten.

Solution

Add optional DKIM configuration fields to the Global Configuration when the mailer is set to PHP Mail or Sendmail, along with a master toggle:

• Enable DKIM (boolean toggle)
• DKIM Domain
• DKIM Selector
• DKIM Private Key (inline or file upload)
• DKIM Passphrase (optional)
• DKIM Identity (optional, fallback to From address)

When Enable DKIM is set to Yes, Joomla should validate the required fields and apply them to the PHPMailer instance during mail sending.

Additional context

This would improve email deliverability and compliance with standards like DMARC, DKIM, and SPF. While SMTP is often configured externally to handle DKIM, PHP Mail and Sendmail require Joomla to provide native DKIM support in order to sign messages correctly.

Providing this functionality will help site owners ensure their transactional and notification emails reach recipients reliably, without being flagged as spam.