mail sending problem in using smtp

in my windows application, i am accessing the smtp of a server, and sending mail using the, i could able to send mail to email address which does not have a .(dot) in the address section. (ie) - i can send mail to, but when i try to send mail to smtp mail sending is failed and and error is thrown.

the error message is:
"Mailbox unavailable. The server response was: 5.7.1 Unable to relay for"
Dinesh SubramanianSenior Software EngineerAsked:
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Dinesh SubramanianConnect With a Mentor Senior Software EngineerAuthor Commented:
problem was with setting the relay server.
Sanjay SantokiCommented:

Based on the error issue most likely seems with the receipt end. Please be sure the email address exist on the appropriate mail server.

Sanjay Santoki
Dinesh SubramanianSenior Software EngineerAuthor Commented:
yes it exists... and it was working earlier also.
I mean sending to email ids of was working earlier....
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Jesus RodriguezIT ManagerCommented:
Check if your SMTP server have the right to relay mails to this domain ( or try to send to another address on the same domain
Jesus RodriguezIT ManagerCommented:
Try to add this line to your code to configure your SMTP Delivery method (2nd line)

  Dim myMailClient As New System.Net.Mail.SmtpClient(AppSettings("SMTPHost"))
 myMailClient.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis
 myMailClient.Send(strFromEmail, strTo, strSubject, strBody)
Dinesh SubramanianSenior Software EngineerAuthor Commented:
problem solved by adding the receipiants mail server address in the relay server
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