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.net mail error - unable to send to a recipient

Last Modified: 2016-01-12
I have a message system on my website and a user was sending email through Message in .net (SmtpClient) to 1000 recipients.  When mail is sent the user goes to a new aspx page that confirms that the email has been sent, but in this case it did not. I also update in sql database that the message has been sent, but in this case it was not updated. I know that at least some received the email.

There was an errormesage in Chrome console:

uncaught Sys.WebForms.PageRequestManagerServererrorexception: Sys.WebForms.PageRequestManagerServerErrorException: Unable to send to a recipient

I know there was at least one email that slipped through detection.  It is something like "t.d.johnjohnsson@someaddress.com".
Can someone explain why this happens and how I can prevent this from happening?
Does .net wait for a confirmation from the SmptClient before proceeding?

I am using .net and C#.

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