Failed deliveries due to one of Cloudflare's SMTP servers being in a blocklist

Hi. This is the error I get:

SPF status


DMARC status


DKIM status


Rejected reason:

upstream ( error: failed to initalize: Unknown error: permanent error (550): 5.7.1 Unfortunately, messages from [] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to Troubleshooting. [ 2023-10-23T23:10:10.956Z 08DBD3B518EC76D6]`

Please help, this makes the email service unreliable.


Unfortunately email forwarding to a server that you do not fully control will always be unreliable. You may be able submit a support request to Microsoft. They are the only ones who can remove an entry from their own blocklist. There is no way for anyone here to know whether such a request will be successful. Depending on how Microsoft operates, the listing may also eventually clear up on its own.

If the email is important, opting for delivery via MX routing directly to a server that you control will always be more predictable.

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