Server at mx1.mxge.comcast.net (2001:558:fd01:2bad::5) returned '421 4.4.2 Connection dropped due to SocketError'

When looking at some support articles online for this issue, it would seem a ticket needs to be opened for the Microsoft 365 email address or the TLS settings need to be updated within the Microsoft 365’s email settings @steved13.

For troubleshooting purposes, are any of the Comcast email addresses that are not receiving the emails sent by the Microsoft 365 account, Comcast email addresses that are yours?

We can definitely troubleshoot issues with not receiving the emails if you have access to any of the impacted Comcast email addresses. Although, this issue appears to require assistance from Microsoft 365 based on the support articles I am seeing for the error messages you are receiving.

I've run the test for our server here: https://testconnectivity.microsoft.com/tests/O365ExchangeDns/input and it passes with flying colors. We don't have any issues with DKIM, SPF or DMARC. Everything has been tripled checked this AM. The office is sending to a total of SIX Comcast.net email addresses in the weekly email. This is how they have been doing it for years. There are other recipients in this email as well. I can see how it might be possibly viewed as SPAM but in this case it is rejected by the Comcast/Xfinity server altogether. If you can get this working we would all greatly appreciate it. Otherwise we will just have to tell parishioners that we cannot send email to them if they have a Comcast.net address. Thank you. 

Hi, @gjames50. I appreciate you circling back. I understand you've had some trouble as well. I see @XfinityAlex provided some feedback, and they are certainly an expert in this area. I see they are checking on this further and I would allow them more time to give you any information they find. You can also visit this link https://spa.xfinity.com/postmaster?faq=avoid-blocks-ipv4 for further assistance. 

And for clarification, hosted M365 exchange with custom domain.  Email threads sent to smaller, non-bulk sizes (we use bulk mailers for messages > daily exchange limits).  Errors same a GJames above:
Server at mx2.mxge.comcast.net (2001:558:fd00:4f::5) returned '421 4.4.2 Connection dropped due to SocketError'

Users sending an email will receive a notification from M365 that is was not delivered, this note is at the end of that message.

To be clear, users seem to be able to fwd the message to users individually - the error happens when more than one comcast.net recipient is included.  If it was DNS/MX/etc it would not prevent us from fwd'ing the very message that was blocked 24 hours before.

This isn't some rate limiter, I don't think.  These emails have 5-30 comcast.net users in it, and our total # of messages sent to comcast are less than 750 in 10 days.  It's very odd.

We don't have the email volume you do. Small church. Also, the church uses Comcast Business for internet and I am also a client of Comcast. So I did send a report in on that side. Never actually heard anything back from that side. So far this forum has been the only activity on the issue.

We had someone who uses the comcast.net email dig in on their side but they have nothing. can't find spam or anything. And when they spoke with someone at Xfinity they pushed the blame back to the church sending the email.

It's bizarre as we hadn't been experiencing this issue until recently. 

I checked all the same stuff you did at MXtoolbox, etc. I even did some editing to clean up the DNS settings a bit. Verified all of these. For a small church the email server is very sophisticated. All hosted by Microsoft. Really there is no excuse for this to not work. Comcast needs to play well with others.

Beyond what we are doing here there is absolutely nothing we can do but wait.

If it fails to come to a resolution we will just tell folks we can't sent to Comcast.net email addresses. The church is too small and the budget is too low to do anything else that adds cost. 

Have a fantastic day!

Found in an SMTP log
https://postmaster.comcast.net/#RL000010

Which is not on one of the error code pages, but I was able to find it on the FAQ page.

"

 RL000010 :: Mail to Comcast is queued and I see RL000010 in my email logs. What does this mean?

Comcast has instituted rate limiting on all inbound email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any systems affected by this rate limit will receive a 4xx message (temp-fail) during the SMTP transaction. This message is designed to instruct the sending server to try again at a later time to deliver its email.

This rate limiting policy is based on historical volumes and quality of that domain's volume. This should apply to both DKIM and SPF.

New domains that are expected to send significant amounts of email to Comcast subscribers should start by sending low volumes of mail, increasing as time goes on. As your domain sends more email, and our systems receive positive feedback from users, its limit will increase daily.

If the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue."

GJames, did you recently adjust DNS?  B/c I did, and I'd bet Xfinity is classifying both of our servers as "new servers" b/c of the DNS updates, even though they are not new and throttling us.  The solution might be to simply wait.

@XfinityAngie​ I did this the same day I posted here. Haven't heard anything back. So, not sure what to say there. 

@gjames50 Send me a DM with the domain of the sending messages.  This shouldn't still be happening.  Thanks

@gjames50 Thank you for confirming the issue is still ongoing. At this time I would recommend contacting our Xfinity Customer Security Assurance team, they would be the best team to assist you with finding a resolution. You can contact them by calling 1-888-565-4329, they are available from 8 AM - 12 AM EST, 7 days a week.

@gjames50​ Our domain also experiencing this, and it appears to have started in the last two months. We are using O365 for our organization and emails to only @comcast.net accounts bounce back. Looks to be exactly the same issue, eager to understand what the problem is.

2/8/2024 5:35:18 PM - Server at <REDACTED>.namprd12.prod.outlook.com returned '550 5.4.300 Message expired -> 421 4.4.2 Connection dropped due to SocketError'
2/8/2024 5:20:40 PM - Server at mx1.mxge.comcast.net (2001:558:fd00:4f::4) returned '421 4.4.2 Connection dropped due to SocketError'

@XfinityBillie​ I just called this number. The support technician said that sending to @comcast.net accounts requires them to be individually enumerated on emails now (no longer able to have them as part of a group). 

That seems like a somewhat daft policy...

Hello @Bargainville! I'd be happy to look further into this issue for you. Please send us a direct message with your first/last name and full address. You can start by clicking the "direct message" icon on the upper right page of the forum page. Once you click on that, input our shared handle "Xfinity Support" to send us a direct message. If you're signed in to your forums profile and because you have already posted to the public page, you should have the option to do so.