We don't recommend that you use the SmtpClient class for new development because SmtpClient doesn't support many modern protocols. Use MailKit or other libraries instead. For more information, see SmtpClient shouldn't be used on GitHub.
For example, SendGrid deprecates support for TLS 1.0 and 1.1 on the 5th of June.
As stated on
System.Net.Mail.SmtpClient
's official documentation page:For example, SendGrid deprecates support for TLS 1.0 and 1.1 on the 5th of June.