This seems to break in the scenario when the headers are stored as a string array (triggered by sending a test email).

Perhaps it would be helpful to mention what plugin is having problems with mail log display?

Thank you for the feedbacks.

I get PHP warnings from this plugin on many sites when logs are turned on (see #114). Those are maintained sites running the latest version of WP Simple SMTP and WordPress, with PHP 8.0 or PHP 8.1 and MariaDB 10.4

I’ll do more tests.

From reading the WordPress Core code on wp mail, see if this works on an unmodified version. I'm still somewhat operating blind until I know what plugin is causing this.

in src/log/class-log.php line 151

/**
* Gets the server dispatch headers.
*
* @return string|string[]
*/
public function get_headers() {
  if ( ! is_array( $this->headers ) ) {
    return explode( "\n", str_replace( "\r\n", "\n", $this->headers ) );
  } else {
    return $this->headers;
  }
}

I tried your mentioned setup in a Docker environment, and I was not able to replicate the issue - sending a test mail, the headers were a string array.

I wanted to come back earlier but had not time to investigate more. My latest test showed a difference between test emails and regular emails sent by WordPress, but both have valid headers (string or array). Then I did a test with Gravity Forms and saw headers saved in an object instead of an array.

Not a worry. I'll do some digging later with your updated info. The snippet I provided might not work anyway due to how the log object conversion occurs, but I've run out of time to debug at the moment.