Send many mail problem

I'm creating newsletter in asp.net. I'm sending to 2000 email recipients. This operation iterate every mail and send using SMTP mail. Please see my previous question .My server use hmailserver.

My problem is

Sometimes this error shown: Mailbox unavailable. The server response was: 5.1.1 Recipient address rejected: User unknown in r开发者_如何学编程elay recipient table

or

Mailbox unavailable. The server response was: Unknown user Mailbox unavailable. The server response was: Unknown user

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You should send mail to each recipient separately. But instead of using Send method use SendAsync method. You can hook SendCompleted so you know which recepient passed a which failed.