I don’t see anything else that looks wrong.
Usually, the &fiarFrom value needs to be for a user who has an email account at the server with the right to send mail.
Since the redirect is working, the AutoResponder is definitely completing and return true.
If you use an anti-spam service like Akismet, that could be a factor.
You might take a look at this.
The only other thing I can suggest is going into the Autoresponder code and adding some debugging statements that write values to the MODX error log like this:
$msg = 'ReplyTo: ' . $emailReplyTo;
$this->modx->log(modX::LOG_LEVEL_ERROR, $msg);
The code is in this file: core/components/formit/src/FormIt/Hook/Autoresponder.php