How to switch between using SHA-2 instead of SHA-1?

My program uses SHA-1 certificate for SSL connection. The SHA-2 certificate has been widely used now by some web services (Gmail) instead. This causes blocking incoming connection to SMTP servers during email notification setup.

To send email I use SmtpClient like this

using (var smtpClient = new SmtpClient(serverSettings.SmtpServerName, (int)serverSettings.SmtpPort))
{
     smtpClient.EnableSsl = serverSettings.SmtpUseSsl;
     smtpClient.UseDefaultCredentials = false; 

     if (!string.IsNullOrEmpty(serverSettings.UserName) || !string.IsNullOrEmpty(serverSettings.EncryptedPassword))
     {
          smtpClient.Credentials = new NetworkCredential(serverSettings.UserName, serverSettings.EncryptedPassword);
     }
                ...
      smtpClient.Send(message);
}

I can't send an email by using this code and I don't want to allow "less secure apps" in my gmail account.

How to implement or switch to SHA-2 certificate for email notifications?

Solution

SHA-1 vs. SHA-2 is completely unrelated to the problem you have. "Less secure apps" are considered for google the application which don't use OAuth 2.0 for authentication (which would allow for 2-factor authentication) but instead only a simple password. See New Security Measures Will Affect Older (non-OAuth 2.0) Applications for more information.

For using OAuth 2.0 with C# see SMTP and OAuth 2