Emails not being sent every minute, and probably overlapping.

[lfcc1]

Hello, I have been using this plugin and it works fine.
However, recently my subscriber's number highly increased, generating this error:

Swift_TransportException: Expected response code 220 but got code "421", with message "421 4.3.2 The maximum number of concurrent server connections has exceeded a per-source limit, closing transmission channel.

When I send the newsletter to all my subscribers, I can see the emails "in queue" number going down almost 1 per second. I dont know why this is happening since I configured the task for everyMinute and without overlapping.

Plugin.php

    public function registerSchedule($schedule)
    {
        $memory = (int)Config::get('indikator.news::memory_limit');
        $schedule->command('queue:work --daemon --queue=newsletter --memory=' . $memory)->everyMinute()->withoutOverlapping();
    }

cronjob -e

* * * * *  path/to/october  && php artisan schedule:run >> /dev/null 2>&1

Any idea what could be causing that error?
Thank you