Xfinity is using Akamai services, and Akamai doesn’t like something about you or your client or your IP address, per:
https://www.reddit.com/r/Comcast_Xfinity/comments/e1pz01/access_denied_to_all_xfinitycom_sites/
(And we don’t know what Akamai is detecting, and neither Xfinity nor Akamai are likely to describe their detection algorithms.)
Previous reports:
https://forums.xfinity.com/conversations/xfinity-stream-website/access-denied-you-dont-have-permission-to-access-httploginxfinitycomlogin-on-this-server/686818b27a19e61f5c2d0981
https://forums.xfinity.com/conversations/email/access-denied-when-trying-to-load-xfinity-website-on-computer/6896a4c2c3884e34159379c5
https://learn.microsoft.com/en-us/answers/questions/2390376/why-am-i-getting-errors-edgesuite-net-18-5494d817
Maybe power-cycling the ISP modem helps, maybe switching off iCloud Private Relay, or removing any add-in VPN apps. Or maybe the firewall block ages out, and communications resume.
But fundamentally, this is an Xfinity issue with an associated service, and not something that anybody around here can help with. I’d ponder moving email services, as ISPs usually aren’t all that keen on running mail services anyway.