we can first try with MonoX admin user, you can open a new one or change the password for some of the existing users and let us know the credentials.
Can you please check the following
1. Take a look at the EventLog on your server (if shared hosting is in place take a look at the shared hosting panel logs)
2. Check if your SMTP requires authentication, and if it is using standard ports
3. As a last resort you can add the following code to on of your pages to see the exact message mail sender is throwing
MailMessage mailMsg =
"Test SMTP server"
"This is a SMTP server test and MailSender test"