An app that sends email notifications to my Gmail account stopped working sometime in the previous months (I wondered why no emails were arriving!). SmtpClient Send was throwing:
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
As usual, 15 solid minutes of web search produces confused, conflicting and preposterous suggestions. Finally someone hinted that they had to change their password to something "stronger", I didn't think that was credible, but when I visited the Gmail Account settings link they mentioned I noticed a suspicious entry titled Access for less secure apps which was disabled. I haven't been to this screen for years, so they had added an extra security hurdle which I didn't know about. Setting this to Enabled allowed my app to send its email without any code changes.
