Error sending email (from xxx to xxx with reply-to not set)

Same error when using the test email in the configuration form :(

PHPMailer SMTP ( https://www.drupal.org/project/phpmailer_smtp → )
It worked for me too! :)
Don't forget to follow its configuration instructions!
(e.q. the https://www.drupal.org/project/mailsystem → installation)

This was added about the same time https://github.com/PHPMailer/PHPMailer/blob/v6.9.1/src/PHPMailer.php#L1581

Switching to phpmailer_smtp also fixed this issue for me. I had debugging enabled but no extra information was visible.

Hello, in my case the problem was that the 'reply-to' email was empty, so i changed the below line (file /src/Plugin/Mail/SMTPMailSystem.php):

from

if ($reply_to_email !== $from) {
  $mailer->AddReplyTo($reply_to_email, $reply_to_comp['name']);
}

to

if ($reply_to_email && $reply_to_email !== $from) {
  $mailer->AddReplyTo($reply_to_email, $reply_to_comp['name']);
}