There is only one recipient.

Yes, all the fields are filled at the front end settings > emails.

I went over and checked all 6 points shown in the link one by one. They are all fine at my site. I also checked the junk mail of the recipient. I tested the SMTP and piping email which is <relay@mydomain.com>. It is also fine.

Still, the recipient does not receive email when he receives new message.