Hello. This might not be the right place to report/inquire about something like this, if that's the case let me know please.
There's what I'm assuming is a typo here: https://git.drupalcode.org/project/opigno_learning_path/-/blob/8.x-1.x/o...
$message['from'] = \Drupal::config('system.site')->get('name');
This is passing the site's name rather than the email address as the "from" field for the email. To fix just requires replacing 'name' with 'mail' instead.
$message['from'] = \Drupal::config('system.site')->get('mail');
This caused us all sorts of issues today trying to have emails not going out as "on behalf of". Not being super familiar with PHP's mail() and sendmail/postfix, and my attempts to fix through postfix configuration failing, I ended up switching to an SMTP module, first drupal/smtp and then drupal/phpmailer_smtp. However this only exacerbated the situation with total failure to send anything as it was erroring out.
The array being sent to Drupal's MailManager::mail was this: array ( 'id' => 'opigno_learning_path_opigno_learning_path_user_subscribe', 'module' => 'opigno_learning_path', 'key' => 'opigno_learning_path_user_subscribe', 'to' => 'firstname.lastname@example.org', 'from' => 'SITE NAME', 'reply-to' => NULL, 'langcode' => 'en', 'send' => true, 'headers' => array ( 'X-Mailer' => 'Drupal', 'Sender' => 'SITE NAME', ), )
This caused it to error out as Headers>Sender (nor From afaik) isn't valid when specified in name only, without an email address.