The mail server won't know that its undeliverable until it tries to send it. "Sending" a message from .NET doesn't really actually send the message. All it does it hand it off to the SMTP server for sending.
An SMTP server can only verify that a email address is bad if that SMTP server is responsible for handling addresses to the email address host. Example:
You have a mail server mail.somedomain.com which handles messages to any email @somedomain.com. If you connect to that mail server and try "firstname.lastname@example.org" that server can deny that mail because it knows that the mail account "emailaddressthatdoesntexist" does indeed not exist. However, if you send mail to "email@example.com om", your mail server won't know whether it's good or bad until it tries to connect to "someotherdomain.com"'s mail server to relay the message. By then, your Send call has completed. In fact, it could be days before the message goes thru as the mail server tries to relay the message depending on lots of factors.
If you are sending the mail FROM a monitored account, then the sending mail server should return a bounce message if it doesn't succeed.