I understand that it’s PHP Mail, but the guide (https://infinityfree.net/support/sending-email-from-your-website-php-mail/) doesn’t make sense to me. Can anyone help?
The most important part of that article are the lines in bold:
At the moment, you can only send registration and activation e-mails through PHP mail(). Outbound e-mails are filtered based on templates for registration, activation and confirmation e-mails. Other types of e-mail, like contact requests, newsletters, notifications, etc. will be dropped and not be delivered to your recipient.
Essentially: most messages are filtered and will not be sent. It’s likely that your messages are never sent either.
You may wish to try sending email with an external SMTP service instead: https://infinityfree.net/support/how-to-send-email-with-gmail-smtp/