If I've list of emailTo address and one of them is invalid.
Does client.Send(email);
returns failure/Exception?
If I've list of emailTo address and one of them is invalid.
Does client.Send(email);
returns failure/Exception?
Yes the SMTPClient's Send(MailMessage) method raises a SmtpFailedRecipientsException exception if one or more recipient addresses were incorrect or unreachable.
Yes and no. For server local addresses (if you send from abc@domain.com to def@domain.com) mail server may return error immediately, but generally speaking mail server processes requests asynchronously and does not return any error if recipient address is invalid.
This is all in the documentation of the SmtpClient class, which you should always read before asking:
When sending e-mail using Send to multiple recipients and the SMTP server accepts some recipients as valid and rejects others, Send sends e-mail to the accepted recipients and then a SmtpFailedRecipientsException is thrown. The exception will contain a listing of the recipients that were rejected.