what is means of this problem when sending email through web-application

Question

i have this error when i sending email through our web application " Mailbox unavailable. The server response was: Requested action not taken: mailbox unavailable or not local"

this is detail of error

System.Net.Mail.SmtpFailedRecipientException was caught
  Message=Mailbox unavailable. The server response was: Requested action not taken: mailbox unavailable or not local
  Source=System
  FailedRecipient=<[email protected]>
  StackTrace:
       at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
       at System.Net.Mail.SmtpClient.Send(MailMessage message)
       at email.Globals.SendMail(String EmailID, String subject, String message, String senderMail) in C:location
  InnerException: 

Answer 1

I would assume that your sender credentials are not in order - if the recipient's mailbox is unavailable, you will get an E-Mail saying so but the actual sending process should work out (if it's not a local recipient).

If I had to guess, I'd say you are trying to send an E-Mail with a sender address belonging to a domain that is not managed by the SMTP server you are using to send the E-Mail (e.g. a GMail address).

Answer 2

It means that the mailbox belonging to "[email protected]" is either not available (it is over its quota or something similar) OR the server is not allowing you to relay emails (which it would have to do if "[email protected]" is not a local domain). It is common for mail servers to behave this way, because it is not desirable for a mail server to act as an open relay.

My best guess is that you problem has to do with the latter. If you have control over the server, you could allow relaying from the server that's hosting your web application.