451 Authentication failed: Could not authenticate [closed]

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