Mostly is an authentication problem. However, sometimes it may just be the recipient server blacklisting yours or it is just an invalid email address.
- Invalid authentication, for instances, wrong password
- Blacklisted or Blocked IP Address
- Invalid recipient email address
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.