451 Authentication failed: Could not authenticate

Question

I am following this link for a test. But I receive

451 Authentication failed: Could not authenticate

at step 4:

4.Enter your Base64 converted API key in the next line as the password.

Does it mean I entered a wrong Base64 converted API key? But I have double checked the key. What's going on?

By the way, I am also using Postfix, and in /var/log/maillog it says

certificate verification failed for smtp.sendgrid.net [198.37.144.225]:587: untrusted issuer /C=US/O=The Go Daddy Group, Inc./OU=Go Daddy Class 2 Certification Authority

Then I followed this link to add the certificate, but I still cannot send the email by Postfix, perhaps the reason is 451 Authentication failed: Could not authenticate ?

Answer 1

Another cause of '451 Authentication failed: Could not authenticate' - because IP not whitelisted

You need to whitelist your NAT or the Public IP of the instance in the Sengrid site then the steps described here would work -

https://sendgrid.com/docs/API_Reference/SMTP_API/getting_started_smtp.html

Answer 2

I've had the same issue. Run the TELNET test again (following the instructions here ), but instead of using the API username apikey and your API key, use your base64 converted login username and password for Step 3 and step 4.

This should return a 235 Authentication successful response. This will mean you've successfully connected.

Given that the username/password test is successful, consider opening your API key Permissions to Full Access and try sending and email. If that works, you can use this link to adjust your permissions for your application.