me too, same problem… the first time I tested the form, last week, there was no problem. But after the first tests, I didn't receive any confirmation emails anymore.
I changed the email address form gmail to the website's… only one came through and then nothing again.
I installed the WP mail SMTP plugin, but the problem is still there… is there anything I should change within the SMTP settings? Here it gets more complex for me...
Thank you!
Nathalie