Issue
When authenticating for SMTP, you may encounter an issue that goes as: 451 Authentication Failed: Could not authenticate. We know that this could be a little scare, but fear not, this is a fairly common issue, and we will be happy to provide the actual explanation behind it.
Product
Twilio Email (SendGrid)
Cause
This is a common issue that always points to an issue with IP access management (IPAM). This is a security feature that only allows an specific IP to access the account.
SMTP users sending via IPs that are not on the Allow List will see an SMTP response of 451 Authentication failed: Could not authenticate.
Resolution
If this is the error, you should check the IP being used to connect to smtp.sendgrid.net and add that IP to the Allow List within the console. This will circumvent the issue.
Also, if you are locked out from your account and can't add a new IP to your account, please reach out to our Help Center Assistant for further assistance
Additional Information
Don't hesitate to check our guide on Email SMTP Service for more details!