The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.

Hello,
While I was trying to send an email from a website using gmail account, I didn't have any issues testing on my local machine but getting to work from production server was a pain.
I was keep getting error message:

"The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required."

3 things will fix it:

1. Go to security settings at the following link https://www.google.com/settings/security/lesssecureapps and enable less secure apps. This will enable google to login from all apps.
2. Enable other ip/timezone for your google account by clicking on following link and click continue to give access: https://g.co/allowaccess
3. Go to account settings 
• Under Recent Activity section, click on "Notifications and Alerts" 
• Now there you will see "Unusual Activity". 
• Select your related event and approve it by clicking "Yes, That was me!".   This way google will add this clear this activity from suspicious list of activities.

HTH