I can't ping that host either, though I can receive email. I suspect that it is configured not to respond to pings. That doesn't mean that it's down, however.
The good news is that you're getting valid IPs for the name. If things are working, you should be able to:
telnet port 993, and have it close the connection the first time you hit the <ENTER> key. This means that you connected to the server, but since you don't speak IMAP, it didn't want to talk to you (it should be much friendlier to your email client)
telnet port 12345 and get connection refused. This means that you can reach the host, but it's not listening on that port.
If the above are true, you can connect to your imap server, but something else is the problem; perhaps your client is misconfigured, and comcast themselves should be experts on setting up most email clients. If you get a connection refused talking to port 993, I don't know what that means. If you try to connect to port 993 and it waits a few minutes before timing out, then something is either dropping your request or Comcast's response. Firewalls and routers are normally configured to only allow traffic on certain ports and drop all requests/responses on other ports, so if you time out, suspect that you have one or the other not programmed to allow port 993 access.
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And I was just able to fix my own problem. I use Ubuntu Linux, so I found a bug report at https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1647031. In my case, I had to install libnss-resolve and that fixed things.
For the vast majority of you running something that isn't Ubuntu Linux, that means that, if your email can't resolve imap.comcast.net and you can't ping it, but you can nslookup it (I think that nslookup is available at the command line for both Windows and Mac), you've got a local resolver problem. Not sure if this helps anybody, but it's what I know.
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My Linux host stopped receiving email on the 28th, though my phone has no problem retrieving it. I see about half of what's going on, but not the whole story.
If I try to ping imap.comcast.net, I get "Name or service not known"; in other words, ping can't even figure out the IP address of the host. Running something like telnet produces similar results, so there's no surprise that my email client can't resolve the name either.
However, if I run "nslookup imap.email.comcast.net", it tells me that imap.email.comcast.net is a CNAME record for imap.ge.xfinity.com, and lists multiple IP addresses for the latter (for the deeply technical, the response also shows that it's querying my systemd-resolved server on my host at 127.0.0.53).
This means that DNS can figure it out, but that programs like "ping" and my email client (Thunderbird) aren't getting their answer from DNS. Comcast can't change how my software looks for those names; all it can do is put the right answer into DNS, and it appears that it has done this.
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