Error when mailing Invoice

I am getting the below error when i try to mail an invoice. I never got it before. Kindly assist

System.Net.Mail.SmtpException: System.Net.Mail.SmtpException: Error in processing. The server response was: 4.3.2 Internal server error
at System.Net.Mail.RecipientCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at ManagerServer.HttpHandlers.Businesses.Business.Email.Post()

I have the same problem today