Please see immediate post above... in addition, a day ago XfinityCSAEmail stated "...the LAN designation has changed on the device causing the issue to

[Edited: "Personal Information"]

"  Would you please elaborate on this?  By LAN designation, do you mean IP Address?  What to you believe would have caused such a change (e.g., a problem with my Netgear Orbi router within my private home network?"

Thank you.

I assure you that I have tried all such solutions that can be googled on the internet.  None of them address the problem nor would explain how my situation of being able to send email during certain parts of the day and not during other parts of the day.

Over the past three days I was unable to send email during afternoon hours, but I noticed after 6:00 pm CST I could.  Therefore, yesterday at 5:59 pm, I tried to send an email and it failed.  I kept pressing the outlook sync button every 5 seconds (not changing anything else), and I got failure after failure until right at 6:00 on the nose the sync worked and email from my outbox was sent.  Email then worked the rest of the evening.  

I suspect this is a security/firewall issue of some sort, but it is not on my laptop or within my private network.  It is either my ISP or Comcast.  I don't know which.  

Can someone please answer the question as to whether there is anything on Comcast's end that would explain the 6:00 pm CST reset that occurred on the last three days.  For example, if a user such as myself with a particular IP address coming into Comcast's site repeatedly has failed authentication, is it possible that Comcast my block that account (or IP) for a certain number of hours and then reset for security reasons? 

I am now working on a new theory that the problem may be with my Windows 10 laptop going into sleep mode with Outlook open.  Two days ago, I deactivated sleep mode on both my laptop and my wife's laptop.  Rather than letting my PC go into sleep mode when not in use, I am now closing Outlook and shutting down my laptop (Power - Shut Down).  I have not had a send/receive error since (first time in months where this has been the case).

I'm not sure what is going on with this, but I have read other threads where people have had problems with Outlook in sleep mode.  Not issues identical to mine, but nonetheless time-out issues of one sort or another after awaking outlook from sleep mode.  I think (but not sure) that Outlook doesn't completely go to sleep in sleep mode.  Instead it continues to send and receive email.  I'm fairly certain of this because I have had email stuck in my outbox in the past because of send/receive errors when my laptop has gone to sleep late in the evening.  Then the next morning, I can see from my Orbi router log that whatever was causing the problem cleared itself and the email was sent at 2:00 a.m. (when the laptop was asleep).  I have also noticed that the first time I login in the morning (awaking from sleep mode), that email received during the night already populates my inbox.  It is not as if a sync occurs to retrieve these emails when the laptop awakes - they are already there. 

So perhaps while in sleep mode, something is going haywire with the authentication process and after so many attempts, some type of block is put on the account for security reasons.  The block stays on for so many hours until it is eventually cleared.  Where this block is occurring is still a mystery, and I plan to do some more testing to figure this out.  However, I'm going to continue to leave things as they are for a few more days and see if things continue to work with sleep mode turned off on our laptops.    

Hi again and thanks for your Reply, as this is a most frustrating issue/problem, and with all of your explanations and things you've tried, Xfinity should be able to figure out what's going on, especially since nothing they've suggested has worked.

Also, and I don't want to discourage you as I applaud your continued attempts to find a solution/resolve this, but both my computer, and screen are set to "never turn off" when plugged in (which it always is), and as I said, I have literally, almost word for word the exact same situation going on at the exact same timeframes that you have so well documented.

Looking foward to hearing the results of your attempts with powering down, good luck!

UPDATE:  I went two days with sleep mode set to "Never Sleep", auto send/receive turned off in outlook, and following a strict regimen of not leaving outlook open when not in use.  I had no email send errors for these two days until yesterday afternoon.  I had turned auto send/receive back ON in outlook and left my PC turned on with outlook open but unattended for a few hours.  When I got back on my laptop around 3:30 pm CST and tried to send an email, I got the 0x800CC0F error.    

After getting the error, I unplugged my Orbi Router from the ONT (an Adtran Fiber 410) and plugged the ONT ethernet connection directly into my laptop, and presto, the email stuck in my outbox was sent.  I reconnected/rebooted my router and tried a second email send through the router via wifi and it failed again with the 0x800CCC0F error.  Did another direct connect to the ONT and the email sent successfully.  Unplugged it, reconnected the router and tried sending through my wifi connection and the send email failed again.  I'm not sure exactly what this means.  First thought is that this may mean that it is not a Comcast issue.  However, I believe when I bypass the router and do the direct ethernet connection from my laptop that Comcast is seeing a different IP address coming at it from the laptop versus the IP address of the router which is blocked.  I think this may be no different than not being able to send on my phone through wifi, turning wifi off, and then sending successfully through my cellular carrier (i.e., diff IP address as explained in previous post).  

At 5:59, I again got on my PC connected via wifi with a stuck email in my outbox and started pressing the outlook email sync button every 10 seconds or so.  I got 8x800CCC0F errors again and again until right at 6:00 pm CST on the nose and then whatever was blocking the email from being sent cleared itself and the email went through.  Email sending then worked the rest of the evening and through this morning as well.  6:00 pm CST happens to be 12:00 a.m. GMT which is interesting time to note.  My ISP swears up and down they have no network processes that would explain the 12:00 a.m. GMT reset.  I doubt seriously whether there is anything within my Orbi RBR750 that would have such a process either.  

I still therefore think the problem is a Comcast issue.  Not the initial authentication failure per se, but the long-term block that is getting put on sending email until 12:00 GMT.  I.e., it may be that there is occasion where an email send may fail.  I don't know how unusual that is.  I do know that when I have my Orbi admin logging function turned on to send me the transaction log hourly, it will on random occasion fail to send successfully (this happens once every two days on average).  But then at the next hour the log sends successfully.  Yet with my outlook email, if I encounter a send error, for some reason I'm getting blocked for the rest of the day until 12:00 am GMT.  

At 9:13 a.m., send mail is still working from my laptop via wifi.  I'm going to let this ride for a while with auto send/receive turned off so as to eliminate that as a variable.  If and when I encounter another email send error, my plan is to switch to other custom DNS server settings and see if this addresses the problem (e.g., Cloudfare, Quad9, etc.).  

Thank you XfinityCSAEmail.  It is extremely helpful in troubleshooting this matter to know that the Comcast block is not 5 min.,1 hour, or whatever, but all day until the block flag is cleared at 12:00 am GMT.   It would also be really helpful to know what is meant by "large amounts of failed authentication attempts" (i.e., how many blocked attempts over what time interval triggers the block flag?), but I appreciate for security reasons Comcast is unwilling to share this.

All indications are from my testing is that the issue is not a "user-sided misconfiguration", but something going haywire in Outlook in sleep mode which includes screen sleep mode or PC power sleep mode - anything that requires a login to the PC after a period of inactivity.  SINCE MY LAST POST I'VE FOUND A WORKABLE SOLUTION, BUT MORE TIME/TESTING IS NEEDED TO CONFIRM.  The solution was to change my DNS Server entries on my Netgear RBK750 Orbi Router from my ISP's DNS server IPs to Cloudflare's (Primary 1.1.1.1, Secondary 1.0.0.1).  Since doing this, I've yet to encounter an email send error.  I would like to hear back from user_16fc88 as to whether this works for him as well.

My theory is that there is a bug in either Windows 10 sleep mode or Outlook when in sleep mode (e.g., perhaps unexpectedly the PC wakes up in sleep mode when I'm logged off my PC and attempts to send and receive email which is then causing the authentication failures on Comcast's end - there are known issues/bugs with Windows 10 in sleep mode).  Whatever DNS servers are my ISP is using do not have the sophistication to detect the authentication issue, and they let the sleep mode requests pass through to Comcast.   After so many failed authentication attempts, Comcast puts up the block flag and I'm blocked from sending email the rest of the day.  Cloudflare's DNS servers do appear to have the intelligence to detect something isn't right when these Outlook sleep mode requests come through from my PC and are blocking them.  Comcast never sees them and the block flag doesn't go up.  

I have a call scheduled with MS level 2 support next week to discuss this.  Also, after XMAS I will likely switch the DNS server settings back to my ISP's and see if the problem returns.

BTW, two days ago when I was blocked from sending email by Comcast, I setup a mobile hotspot and was able to successfully send email through it (different IP as has been discussed previously).  I then let my PC go into sleep mode for two hours (longer than the normal time when I would get the Comcast block through my ISP), and when I got back on my laptop, I was still able to send mail successfully (i.e., apparently Verizon's DNS servers also have the intelligence to block any sleep traffic they see from going to Comcast).  This is what led me to switch the DNS settings on my router to Cloudflare's and I have not had a problem since.  

More time is needed to see if the Cloudflare solution is a long-term fix.  I will post again in a few days.

I was having the same problem this morning and tried everything suggested here and nothing worked.    I went into my account settings in outloook, didn't change anything, just saved and now email is working again - I hope this helps

In my case, when I first changed my router settings from my ISP to Cloudflare, I did it when I was currently blocked from sending email by Comcast.  Changing the router settings did not remove the block at the instant I changed them.  However, when Comcast removed the block that evening at 12:00 a.m. GMT while maintaining the Cloudflare DNS server settings, I have not had a send email error since.  

User_521d8c, have you had more issues.  I just found this thread and I am experiencing the identical problem.  Would love to know if you have had success

Thank you. I have experienced the same problem & my conclusions are the same as yours!

 Apparently, Comcast has employed software to limit or bind such access.

I am reluctant to change as I have many worldwide medical contacts from my voluntary overseas service. Frustrating, but I get up early to send & receive the emails & I warn my email recipients that replies may be slow in arriving!

Since my last posting on 12/24, I have not had a single email send error (i.e., everything is working as it should).  What is odd, however, is that on 12/28, I switched my router settings back from Cloudflares' DNS IP addresses to my ISP's default settings to see if the email send errors would return so as to confirm this was in fact the issue  (i.e., it was under the ISP's default DNS server IP addresses that I had the problems initially).  The send errors did not return.  I have kept my router settings to the ISP's default DNS IP settings since 12/28 and have yet to have another email send error.  

I am not sure what this means.  I've had many communications with Comcast and my ISP since my issues began.  Perhaps one of them finally made some changes to correct whatever was causing my problem and has not bothered to tell me because they both swore up and down the issue was not on their end.  Maybe the change was account specific if others are still having the same issue since 12/24.  Nonetheless, I am no longer experiencing email send errors and I am continuing to leave my DNS settings set to my ISP's.  

If the errors return, I will post again indicating so.  

I have had the same experience the last few days that you all are describing, with two separate comcast e-mail accounts, on two separate computers, both with Outlook interfaces. I am still able to receive messages, but unable to send out messages.  I sent test messages out during the day today, which were blocked and sat in the Outbox.  Suddenly, at 7:00 pm EST this evening, they were accepted by the Comcast server and sent to their destination.  This has been extremely frustrating!!!  I am dead in the water in these accounts.  Clearly, Comcast has done something to their system that makes it extremely difficult-to-impossible to interface with their servers.  I have had these accounts for over 20 years, but am seriously considering moving to another host for my e-mail accounts, such as Gmail.  Comcast has become intractable, and they are trying to blame it on all of us.  Just read the responses from the Comcast expert.  I admire the patience you all have, but there comes a point where it is time to ditch a user-unfriendly e-mail host!!!

After reviewing this whole chain, I have decided not to waste any more time on this Comcast e-mail platform.  My time is too valuable.  I, like many, hate the thought of having to establish a new e-mail address with all the contacts I have made throughout my personal life and career.  Comcast has proven they are unreliable, and cannot be relied upon to address customer concerns and complaints on an issue that is absolutely critical for any of us who depend on e-mail for both personal and business communication on a daily basis.  I am starting the process today, setting up new accounts on another platform.

Did you ever find a solution (other than use gmail)?  I've experienced the same issue for several months -- also a former subscriber keeping my comcast email address I've had for 20 years.  Comcast is no help. I've tried most/all the "solutions" discussed with no luck.  Only that at 6:00 pm CDT, messages magically send.

My current ISP could not offer any insight.

By the way, I can send gmail from same pc, using same router, etc. flawlessly.