How to customize Swift_Message creation via Monolog SwiftMailerHandler. See discussion https://github.com/Seldaek/monolog/pull/497

MonologErrorEmailPrototype.php

<?php

namespace My;

class MonologErrorEmailPrototype
{
    private $fromAddress;
    private $toAddress;
    private $host;
    private $env;

    public function __construct($fromAddress, $toAddress, $host, $env)
    {
        $this->fromAddress = $fromAddress;
        $this->toAddress = $toAddress;
        $this->host = $host;
        $this->env = $env;
    }

    public function buildMessage($content, array $records)
    {
        $message = new \Swift_Message();

        // Do any dynamic processing right here create Swift_Message given $content and $records

        $message
            ->setTo($this->toAddress)
            ->setFrom($this->fromAddress)
            ->setSubject($this->buildSubject())
            ->setContentType('text/html')
        ;

        return $message;
    }

    private function buildSubject()
    {
        return sprintf('[%s][%s] Error Occurred', $this->host, $this->env);
    }
}

services.yml

services:
    my_error_email_prototype:
        class: My\MonologErrorEmailPrototype
        arguments: ['%mailer_from_address%', '%monolog_delivery_address%', '%router.request_context.host%', '%kernel.environment%']