catch (SmtpException oSMTPException)
{
    string sFailedEmailIds = string.Empty;
    string[] sMailids = null;

    ((System.Net.Mail.SmtpFailedRecipientsException)(oSMTPException)).
        InnerExceptions[0].FailedRecipient
}

string sFailedEmailIds = String.Join(",", ((System.Net.Mail.SmtpFailedRecipientsException)(oSMTPException)).InnerExceptions.Select(x=>x.FailedRecipient));