421 4.3.0 Upstream error


Recently, we have started seeing the following errors come through from people attempting to email our email address:

421 4.3.0 Upstream error, please check Postmaster · Cloudflare Email Routing docs for possible reasons why.

When looking at the address provided, I do not see any specific messages suggesting how to fix the issue.

This seems to only happen with the user is sending a message from Gmail accounts. When a user responds, there message seems to send as normal but it never arrives to us. That user will then receive a bounce back including that error message maybe 24 hours later and then eventually, the email might send through to us. Whilst I understand that this is a temporary message, I have had reports from multiple contacts stating that this is happening and I am not receiving messages (as far as I am aware), for up to a day, which obviously is not a good transfer time.

In CF, we have the default CF MX records set up, along with two for Mailgun, mxa.mailgun.org and mxb.mailgun.org.

Is this an issue with Cloudflare or Gmail and where should I look to determine where the issue may be?


For the same hostname? That would be a no-no.

Also, if you have Mailgun, you shouldn’t need Cloudflare Email Routing at all.

Can you provide a screenshot of your Cloudflare DNS screen so we can check the records for you?

1 Like

We’re getting this exact same issue, only one MX record on our domain though.

Ah, makes sense

Cloudflare is currently listed in Spamhaus. Effects of a single point of failure after they moved their email domain to a fresh new one without any age.

As such, the entirety of email routing is down.

Not true. I am receiving mails as usual. With a few rejected by google. But 95% just getting into my gmail inbox without an issue.

most smtp routing errors resolved but still receive failures due to throttling. i have now abandoned cloudflare as horribly unreliable and embarrassing. cloudflare outage still not 100% even after more the 48hr of being down.