Hey there, @WNM4, thanks for reaching out through Xfinity Forums regarding your email account. We would be happy to help with troubleshooting! I would like to take a look at your account. 

Please send us a direct message with your name and service address, so we can assist you further. To send us a Direct Message, you will need to sign in to the Forums if you are not. Then you will need to locate and click on the "Direct Message" icon located in the upper right corner of this page. Next, click the "New Message" (paper & pencil) icon and type "Xfinity Support" in the to line, and select "Xfinity Support" from the drop-down list. You can proceed to type your message in the text area near the bottom, and press enter to send the message. Please let me know if you have any questions.

I'm having the exact same issue!  Began overnight - email worked fine on the 13th and without changing any computer setting, starting on the 14th began receiving sending comcast error 0x800CCC0F using Outlook/Microsoft 365 from laptop and on iPhone. I have 2 Comcast emails and having the same problem. I can send/receive other emails (gmail, work, etc). I can send/receive comcast emails on a Browser, but not through Outlook.

I have now spent 10-12 hours trying different things and reading a lot on the internet.  Nothing has changed in 3 days.

my summary

email settings are the same as above.
verified that permission given for using 3rd party app https://www.xfinity.com/support/articles/third-party-email-access
I've turned off firewall and virus protection-no change
I uninstalled the last Windows update - no change
and probably other things I'm forgetting

First call to Xfinity.  Basic troubleshooting and then ran out of options "we'll call you back in 4 hours".  No call back.  4 hours later, I called again.

2nd call to Xfinity.  More basic troubleshooping and then ran out of options. Total call length 1 hr 16 minutes. Got a case # "we'll call you back in 72 hours".  No call back.  71 hours later, I called again.

3rd call to Xfinity.  After 4 calls, I finally reached tech support (first 3 calls I was erronously routed for a phone explanation on how to set my default language-TV?). Did more basic troubleshooting. I asked if my email was being blocked due to spam - Xfinity hasn't responded. total call length 53 minutes. I'm supposed to get a call back in an hour. 

Hello @frustrated_user34! We appreciate you taking the time to reach out to our Team on Forums regarding this email issue. I am sorry to hear about the issues you have experienced trying to work with us in getting this investigated. 

I also apologize for any inconvenience or frustration this has caused you thus far. My team would be more than happy to further assist you. Please send us a Direct Message. Here are the detailed steps to direct message us: 
 • Click "Sign In" if necessary
 • Click the "Direct Message icon” (upper right corner of this page)
 • Click the "New message" (pencil and paper) icon
 • Type "Xfinity Support" in the To line and select "Xfinity Support" from the drop-down list
 • Type your message in the text area near the bottom of the window
 • Press Enter to send your message

No solution yet.  I've been told that my issue has had an "advanced repair ticket" created for it but I've seen nothing further beyond that.  

thanks.  Also I have an "advanced repair ticket" with Xfinity. 

my Outlook error is "reported error (0x800CCC0F): The connection to the server was interrupted.  If this problem continues, contact your server administrator or Internet service provider (ISP). 

My Thunderbird error titled "Login to account comcast failed" and error " login to server smtp.comcast.net with username  _____ failed.

I'm having the same problem: can receive emails, but not send emails.  True on all 3rd party apps: Outlook, thunderbird email, and my iPhone. I have my XFinity box checked to allow for 3rd party apps.

I'm currently on my 5th call to Xfinity. They are checking things on their end.  My previous tickets were marked as 'resolved'  So so so so wrong

@frustrated_user34​ 

I'm pretty sure this has to do with TLS 1.2.  Are your email programs up to date with TLS 1.2?

TLS might be a clue.  no one has asked about that.

Wondering where would I check for TLS 1.2 in email programs?  Outlook?  in Thunderbirder?  on iPhone?

However, Thunderbird was downloaded today, so I assume everything was updated?  Wouldn't outlook (office 365) already have offered updates?

OK,  my full error message is:

Task '@Comcast (Name) - Sending' reported error (0x800CCC0F) : 'The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP).'

No mention of TLS 1.0 or 1.1.    

Looking at my Internet Options/Security on Windows 10 -- Only TLS 1.2 is checked.

@WNM4​ @frustrated_user34 

It looks as though @XfinityAlex will have to chime in here.