SMTP Error! The following recipients...

Most of the clients after their registration/payment they see the above message after confirmation.
But i receive their payment/registration without any errors.

Any ideas?

Hi Panos

From the error message above, it states that the system could not send email to target email address for some reasons

As the email is handled by mail server of your hosting provider, I guess the mail server could not deliver email to that email address. Try to contact your hosting provider, give them the error message. They should understand the error message and help checking it

Tuan