Gmail 5.5.1 Authentication Required
Gmail ratcheted up security options for folks wanting to send email using their SMTP options from another program like 3D. Typical Gmail account settings used in 3D often look something like this:
SMTP Server: smtp.gmail.com
Login Name: This email address is being protected from spambots. You need JavaScript enabled to view it.
Password: yourgmailpassword
Email User Name: Your Name
Return Email Address: This email address is being protected from spambots. You need JavaScript enabled to view it.
Port: 587
Use SSL: Enabled
However even with correct settings, one might get this error returned from the gmail server:
"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"
This means Gmail requires you to do some extra things before they will "allow" you to send email from an outside program through their server. Here's the steps known to work at this time:
Log into your gmail account on the web at www.gmail.com
Click Gear>Settings
Click "Accounts and Import" at the top
Under "Change account settings", choose "Other Google Account settings"
Under Security on the left, scroll down to “Less secure app access" and choose "Turn on access"
When you save your setting and log out of your gmail account, it should now allow third-party apps to be used to send from your account.
Note that if you are using the paid Google Apps for Business, it appears Google Apps has different setup steps than for their free gmail: https://support.google.com/a/answer/176600?hl=en