Host = "smtp.gmail.com";
Port = 587;
UserName = メールアドレス
PassWord = ２段階認証で発行したパスワード

var emailMessage = new MimeMessage ();

emailMessage.From.Add (new MailboxAddress (UserName, From));

emailMessage.To.Add (new MailboxAddress (To));

emailMessage.Subject = Subject;

emailMessage.Body = new TextPart ("plain") { Text = Body };

using (var client = new SmtpClient ()) {


    await client.ConnectAsync (Host, Port, SecureSocketOptions.StartTls);
    await client.AuthenticateAsync (UserName, PassWord);
    await client.SendAsync (emailMessage);
    await client.DisconnectAsync (true);
}

MailKit.Security.AuthenticationException: AuthenticationMechanismTooWeak: 5.7.14 <https://accounts.google.com/signin/continue?sarp=1&scc=1&plt=AKgn
sbuI
5.7.14 3AKy1Iwk847IsaZtJndAdUUVoKz9dQM_63FknYiAi78f5TvEKQtOhEdSMaNLPhYgNPlsXu
5.7.14 rdwyBSf2LzU8FjqJL_4FSQ5wbMuy_xM5wL3utr1ZYTj28c2NwWUtjF1iGxpFPe6xvWzDsr
5.7.14 MpmoyfFLllCmCS8N9c2uvD8brZ47n4F3k0ig5NpdgUHUJjYjED4HM5qw3BAZzqMJgwomaX
5.7.14 Q1UVXGmGSrhnK7EkHNzTuRXh7JF8uflp-RRhBV__iZRXPw-F6M> Please log in via
5.7.14 your web browser and then try again.
5.7.14  Learn more at
5.7.14  https://support.google.com/mail/answer/78754 g123-v6sm35799149pfc.67 - gsmtp ---> MailKit.Net.Smtp.SmtpCommandException: 5.7.14 <https://ac
counts.google.com/signin/continue?sarp=1&scc=1&plt=AKgnsbuI
5.7.14 3AKy1Iwk847IsaZtJndAdUUVoKz9dQM_63FknYiAi78f5TvEKQtOhEdSMaNLPhYgNPlsXu
5.7.14 rdwyBSf2LzU8FjqJL_4FSQ5wbMuy_xM5wL3utr1ZYTj28c2NwWUtjF1iGxpFPe6xvWzDsr
5.7.14 MpmoyfFLllCmCS8N9c2uvD8brZ47n4F3k0ig5NpdgUHUJjYjED4HM5qw3BAZzqMJgwomaX
5.7.14 Q1UVXGmGSrhnK7EkHNzTuRXh7JF8uflp-RRhBV__iZRXPw-F6M> Please log in via
5.7.14 your web browser and then try again.
5.7.14  Learn more at
5.7.14  https://support.google.com/mail/answer/78754 g123-v6sm35799149pfc.67 - gsmtp
   --- End of inner exception stack trace ---
   at MailKit.Net.Smtp.SmtpClient.AuthenticateAsync(Encoding encoding, ICredentials credentials, Boolean doAsync, CancellationToken cancellationTok
en)