I have a problem (see Event Log Message below) that happens every time a user registers.
The user sees an exception error after submitting their registration.
Their registration IS entered into the USER table in the database.

I have checked the settings under Host Settings > SMTP Server Settings and they are correct according to my hosting provider - the test send works.

Yet it appears that either the e-mail to the user confirming their registration OR the e-mail to the Admin (who's e-mail address is also correct) is failing.

Any ideas?  See below:

Message: DotNetNuke.Services.Exceptions.ModuleLoadException: Mailbox name not allowed. The server response was: sorry, your mail was administratively denied. (#5.7.1) ---> System.Net.Mail.SmtpException: Mailbox name not allowed. The server response was: sorry, your mail was administratively denied. (#5.7.1) at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response) at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendEmail(String fromAddress, String senderAddress, String toAddress, String subject, String body) at DotNetNuke.Services.Mail.Mail.SendMail(UserInfo user, MessageType msgType, PortalSettings settings) at DotNetNuke.Entities.Modules.UserModuleBase.CompleteUserCreation(UserCreateStatus createStatus, UserInfo newUser, Boolean notify, Boolean register) at DotNetNuke.Modules.Admin.Users.ManageUsers.UserCreateCompleted(Object sender, UserCreatedEventArgs e) --- End of inner exception stack trace ---