Clear the reply-to address and retry.

If the mailer isn't erroring it's being stopped at the mail server not in PHP. Am looking into removing our PHPMailer as it's a bit of a leftover from Joomla 1.x days that we don't really need anymore. I do not have a timeframe when that will be done. I suggest contacting your host and seeing if they can check mailer logs. It's possible your host might also recommend SMTP over PHP Mail. I don't have anything further to suggest at this time.

It's not caused by PHP 8. My development environment has been PHP 8.1 for months and 8.0 for months prior to that where the email is working perfectly fine. You can try using SMTP within Joomla's mailer settings and CB will use that as well. There is not a way to configure CBs mailer separately.

I've already a pending merge request to switch CBs mailer entirely over to Joomla's. Once reviewed and merged (we use peer review in our team) there won't be a CBPHPMailer anymore. This likely won't be until sometime next week as Beat is taking some well deserved rest this week.