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user_52t1la
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Friday, January 16th, 2026 4:30 PM

emails sent to comcast accounts being rejected

As the President of a small non-profit organization, I use a Microsoft supplied email account that ends in .onmicrosoft.com for company business.  Emails sent from this account are being rejected with the following reply:

resimta-a2p-650768.sys.comcast.net rejected your message to the following email addresses:

xxxxxxxx@comcast.net
Your message was rejected by the recipient email server. Please check the recipient's email address and try resending your message or contact the recipient directly. For more tips to help resolve this issue, see DSN code 5.1.0 in Exchange Online - Office 365. If the problem continues, contact your email admin.

resimta-a2p-650768.sys.comcast.net gave this error:
<xxx@obinh.onmicrosoft.com> sender rejected

This has happened with messages sent to one of my Directors, one of my volunteers and even my personal Comcast email address.

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4 hours ago

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XfinityThomasD

Official Employee

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1.6K Messages

19 days ago

 

user_52t1la Thanks for creating a post. For these types of email issues, I would recommend speaking with our Customer Security Assurance team (https://spa.xfinity.com/help/postmaster), where you can review a FAQ for postmaster issues or report an issue to have their team investigate further.

 

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user_52t1la

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I appreciate the answer.  I had difficulty answering technical questions that I do not know how to answer.  Why not simply let me submit the error report that I received?  

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16 days ago

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user_52t1la

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I eventually figured out the terminology and submitted a report.  The answer that I got back was that the IP address was not being blocked.  Apparently, the problem is that the domain is being blocked by Comcast.  So far, I haven't found where to submit for resolution of a blocked domain.  

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2 days ago

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user_ztrl3z

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I'm having the same issue, as my firm cannot send emails to any comcast.net email addresses, but our IP addresses have no issues.  Rather it seems that our domain is being blocked by comcast, but we're a small local firm, so this doesn't make any sense. How do we request that comcast unblock our domain?

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2 days ago

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XfinityMatthew

Official Employee

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1.4K Messages

The link provided above is the main online option for submitting any requests with that team. Otherwise, calling our main customer service number at 1-800-934-6489 and asking for Customer Security Assurance is the other option. If you are having issues with both of those options we can reach out to our contact within that team to open a request. But typically the online and phone options are the quicker resolution. 

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
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5 hours ago

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XfinityJohnG

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2.4K Messages

Have you been able to contact the security team about that blocked domain issue?

I am an Official Xfinity Employee.
Official Employees are from multiple teams within Xfinity: CARE, Product, Leadership.
We ask that you post publicly so people with similar questions may benefit from the conversation.
Was your question answered? Please, mark a reply as the Accepted Answer.tick

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4 hours ago

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