GMAIL server replies with 550-5.7.26 This mail has been blocked because the sender is unauthenticated

When trying to send an email the recipients mail server replies with:

This mail has been blocked because the sender is unauthenticated

- this happens only when sending via Nylas.

Starting February 1st 2024, GMAIL have enforced DKIM / SPF to ensure they can verify the sender.

Reason: Most providers have two sending servers (SMTP) one for their mail clients another for third party applications such as Nylas. The bulk sending server may not have been included.

Solution: As per the Google linked article in the bounce response, have the customer reach out to the provider to determine what IP's to add to the SPF /DKIM records for their non webmail SMTP servers.

Example of SPF record from How to build SPF in 5 steps

Start with v=spf1 (version 1) tag and follow it with the IP addresses that are authorized to send mail. For example, here’s what the basic SPF record syntax looks like: v=spf1 ip4:1.2.3.4 ip4:2.3.4.5

Resources

February 1st Google enforces SPF /DKIM

Google document linked in response

Updated February 26, 2024 14:09