We have been experiencing an outage on our incoming SMTP servers, your messages could be rejected with the following error:
“451 4.3.0 Error: queue file write error”
Our technical team is already working hard to resolve this issue, based on the best SMTP practices, when a SMTP server returns a 4.x.x error, you should put back your email on a queue, and retry sending later.
We will update this page as soon as we have more information.
If you have any urgent questions, you can contact our Customer Support team here: https://www.mailjet.com/support/ticket/