Issue DKIM signing fails for website-generated emails

[tickteam-daryl]

Server operating system version

AlmaLinux 9.8

Plesk version and microupdate number

18.0.81 #2

I'm trying to resolve an issue with DKIM signing on a Plesk server and wondered whether anyone has encountered something similar.

Environment

• Plesk Obsidian 18.0.81.2
• AlmaLinux 9.8
• Postfix
• Laravel with Symfony Mailer 6.4.46

The problem

Emails sent by our Laravel application are delivered successfully, but DKIM signing fails. The mail log reports:

Sendmail (...) -> signing
DKIM error: DKIM Syntax error
As a result, the messages reach the recipient without a valid DKIM signature, causing email authentication issues.

What we've established

• DKIM signing is enabled in Plesk.
• The DKIM signing handler is enabled and registered.
• The private key exists and passes OpenSSL validation.
• A manually submitted test email from SSH is successfully signed and passes DKIM verification.
• Emails generated by the Laravel website fail with the syntax error above.
• Restarting Postfix and running plesk repair mail -y have not resolved the issue.
• No newer mail-driver package was reported by the package update check.

Our Laravel application uses Symfony Mailer's Sendmail transport, configured to invoke Sendmail in -t -i mode. -bs mode does not work.

I also noticed the Plesk Obsidian 18.0.79 changelog entry stating:

Emails sent locally via /usr/sbin/sendmail are now processed with DKIM signing applied.

As we're running a later Plesk version, I would expect this functionality to be available.

Has anyone experienced this behaviour, or can anyone suggest what might cause Plesk's DKIM handler to reject application-generated messages while successfully signing a manually submitted message?

It's causing deliverability issues with Microsoft email clients - but at a loss to explain why even though we've enabled it, Plesk just refuses to sign the message.

Any ideas would be appreciated.