VPNs are often used by people who send spam and other abusive material, "poisoning" the VPN's IPs.
What does the error message say? Do the two replies at https://forums.xfinity.com/t5/Xfinity-Connect-App/Cannot-send-email/m-p/3297549#M3939 help?
This is the message I get: An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-po-04v.sys.comcast.net resomta-po-04v.sys.comcast.net CSI IP 126.96.36.199 is not permitted to send messages. Please contact support if you feel this is in error.. I had found a workaround on this forum where I entered the ip for me and Comcast. It worked once, when I restarted my computer I was back to the original problem. If I turn off VPN everything is fine. Using Avast Secure VPN, it doesn't have any "split Tunnelling" option or any place I can remove Comcast from the VPN feature......Paul