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marty39
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6 Messages

Friday, December 19th, 2025 5:31 PM

Undelivered Mail Returned to Sender

Starting some time yesterday morning, emails that are forwarded by rules from iCloud mail have been returned to sender. I get an email from iCloud with subject "Undelivered Mail Returned to Sender" with a lot of unreadable text starting with

<REDACTED@comcast.net>: host mx1.mxge.comcast.net[96.102.18.146] said: 530
   5.1.0 <ALSO-REDACTED@icloud.com> sender rejected (in reply to MAIL FROM
   command)

This just started yesterday morning. Forwarding was successful until then. 

It seems to have been corrected a few minutes ago.

What happened, and will it happen again?

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26 days ago

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XfinityChristy

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26 days ago

 

marty39 Hello and thank you for reaching out via our Xfinity Community Forums. It appears you experienced a temporary security block between iCloud and Comcast. The error "530 5.1.0 Sender Rejected" means Comcast’s filters temporarily refused mail originating from iCloud’s forwarding servers.

 

What Happened?

  • Spam Protection: When iCloud forwards mail, it can look like "spoofing" to Comcast. If the original sender has a low reputation, Comcast may temporarily block the incoming iCloud server to prevent spam.

  • IP Greylisting: Comcast’s automated systems likely flagged a specific iCloud IP address. Since service has resumed, the block was likely cleared automatically once the system verified the traffic was legitimate.

Will it happen again?

It is possible, as automated filters are constantly updating. To protect your mail:

  1. Keep Copies: Ensure your iCloud rule is set to "Forward and Keep a Copy" so no mail is lost if a block recurs.

  2. Whitelist: Add your iCloud address to your Comcast/Xfinity address book.

If this persists, contact Comcast Security Assurance at https://internetsecurity.xfinity.com/help/report-abuse/ to request a manual whitelist.

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marty39

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Thanks. I understand this was a temporary overreach by the security system, and it isn't guaranteed not to happen again. I guess it was an IP greylisting error, not spam blocking, because the original sender for some of the messages was The New York Times, and it's hard to imagine that they have "a low reputation."

But I don't see how the suggested means of protection would help in this case. I kept a copy automatically. The problem was that I didn't know that the forwarding had failed until I carefully parsed the error message. It's not easy to find the information in the error message that tells which email message was not delivered. I know all that stuff has to be there so that an expert can figure out exactly what happened, but maybe the part that can be understood by an ordinary user could be put near the top ...?

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26 days ago

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