Setting the Nice Name in emails sent through Swiftmailer

function MYMODULE_mail($key, &$message, $params) {
 $options = array(
   'langcode' => $message['langcode'],
 );
 switch ($key) {
   case 'my_mail':
	 
	 if (isset($params['cc'])) {
		 $message['headers']['Cc'] = $params['cc'];
	 }
	 if (isset($params['bcc'])) {
		 $message['headers']['Bcc'] = $params['bcc'];
	 }

     $message['subject'] = t('@subject', ['@subject' => $params['subject']], $options);
     $message['body'][] = $params['message'];
	 
     break;
 }
}

I guess you already have hook_mail() function defined, but if not, this is the code:

When testing the code above, I was unable to set the headers properly. This is probably due to the fact, that Swiftmailer sets the headers automatically and can not be overriden in HOOK_mail() function.

So using the HOOK_mail_alter() function solves the problem:

use Drupal\Component\Utility\Unicode;

function MYMODULE_mail_alter(&$message) {
	
  if ($message['id'] == 'MYMODULENAME_MYMAILKEY') { // Typically something like "mymodule_my_mail" in this setting
	  
	$senderName = 'My nicename';
	$nicename = Unicode::mimeHeaderDecode('"' . $senderName . '" <' . \Drupal::config('system.site')->get('mail') . ">");
	 
	$message['headers']['From'] = $nicename;
	
  }
}