If you are having this issue it's because the imap.comcast.net server is not resolving, they must be having a DNS issues somewhere. You can confirm this by going to a command prompt and typing "ping imap.comcast.net" (without the quotes) and it will say there is a problem resolving that name. I got around it using the IP address 18.104.22.168 as the server name so the mail client hits that server directly. However this isn't a good solution as IP addresses change. You can use "nslookup imap.comcast.net" for a list of other IP addresses if this one doesn't work for you.
Dear comcast, this is a serious issue and needs attention ASAP.
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