"The server response was: Syntax error, command unrecognized"

This is the error I get when I try to create a user using the CreateUserWizard control. The user is created, but the error seems to occur when the application tries to send a notifying email to the just-created user's email address, containing the automatically-generated password.

 Here is the relevant (topmost) portion of the stack trace:

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 System.Web.UI.WebControls.LoginUtil.SendPasswordMail(String email, String userName, String password, MailDe ...