Emails not being forwarded to

For the past few days, forwarded emails are mostly errorring out with Delivery Failed to address. Only one in about 20 emails gets through and there is no pattern to which ones are not being delivered even with retries.

The error is similar to

upstream ( temporary error: Unknown error: transient error (451): 4.7.500 Server busy. Please try again later from [2a01:111:e400:fc4a::49]. (AS750) []


I too use Cloudflare email routing to forward emails from my personal domain to my email address. Until a couple days ago it has worked flawlessly.

Recently I suspected something was wrong as it is very unusual to go more than a couple hours without any new emails and I hadn’t received anything for half a day.

I logged into my Cloudflare account and checked email routing and saw hundreds of delivery failures just like @proteus1 reported.

Usually this error message indicates that Microsoft has observed a change in pattern and has greylisted Cloudflare’s mail servers. Perhaps Cloudflare added new servers or changed IP addresses? I suspect there is nothing Cloudflare can do about this as the error is coming from Microsoft’s servers.

To answer my own question I just read the blog update for Email Routing leaves Beta which says that Cloudflare has just enabled IPv6 email egress. Thus Cloudflare’s mail server’s IP address did change and Microsoft detected that change and grey listed Cloudflare.

Hopefully this will clear up soon once Microsoft removes Cloudflare’s mail servers from the grey list.


Is there any documentation on how the email forwarding service works as it pertains to delivery errors? I know it doesn’t support NDRs, but are these type of errors reported back to the originating SMTP server, so that e-mail delivery can be retried later?

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Getting the exact same error here, started as soon as it left beta. Hoping this is sorted soon

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