Mostly is an authentication problem. However, sometimes it may just be the recipient server blacklisting yours or it is just an invalid email address.

Causes

• Invalid authentication, for instances, wrong password
• Blacklisted or Blocked IP Address
• Invalid recipient email address

Fixes

Step 1: Email Address Correct

Ensure the email address is correct and properly spelled.

Step 2: Correctly Authenticating

Ensure the email user is correctly authenticating. For example, login and password.

Step 3: IP Address Whitelisted

Ensure the IP Address is not blacklisted by the recipient.