This incident has been resolved.

A fix has been deployed, our SMTP servers are back online with full capacity. We are still monitoring the situation to make sure everything is stable.

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/