I recently started using a vpn service. When trying to send an email from Thunderbird, the server responds with an error message that my IP address is not authorized to send emails. This continues to happen even if I shut down the vpn client. I do not have this problem when using the Xfinity Connect web interface from Firefox on the same computer.
Here is my understanding of what is going on:
Comcast is blocking SMTP service requests that originate from IP address of sources that are associated with high levels of spam. VPN services are favored by evil-doers/spammers since their identity is cloaked and are therefore blocked.
When you use Thunderbird (or other email client), your email (SMTP) request is encrypted by the VPN client on your computer, sent to the VPN server, and then that (VPN) server sends it to Comcast but associates its IP with it, rather than using your local IP. Now the request appears to come from an IP address that has been associated with frequent spam, so Comcast blocks it. When you shut off your VPN, your local IP address is used and the SMTP request goes directly to Comcast - if you are not a spammer, Comcast will allow SMTP requests originating from your IP.
There is no solution to this that I know of - unless Comcast decides to use some other blacklisting method that does not simply use IP addresses to identify evil-doers/spammers.
I assume that one can use Comcast's web-based email client even with your VPN on because all such traffic is sent as HTTP service and is not blocked - only SMTP service is blacklisted.
This basically my humble understanding and if someone can correct me, please do.