.net mail error - unable to send to a recipient

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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Umar Topia.Net Full Stack DeveloperCommented:
SMTPClient will not throw an exception in case an email is valid (especially the domain name... i.e. xxx@google.com)... and moves ahead. However, in case of issues with SMTP Server or the mail server being unreachable .Net would get an exception from SMTP server and stops the further execution.

In case you are sending bulk emails at the same time, you also need to check if there is any limit being hit somewhere which is causing this behavior.

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