Error occurred while sending mail: Outgoing server (SMTP) error

I recently installed a VPN desktop app on my Windows 11 PC. I use Thunderbird as the email client app on my PC. With the VPN deactivated, I successfully receive emails via Thunderbird and can also successfully send emails. When the VPN is activated, I successfully receive emails via Thunderbird, but cannot send emails. When I try to send an email, I get the following error message: "Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-h2p-555059.sys.comcast.net resomta-h2p-555059.sys.comcast.net ESMTP server not available (212238/143.244.47.90)." Again, Thunderbird can send emails when the VPN is deactivated but cannot send emails (resulting in the above error message) when the VPN is activated.

Does anyone have any suggestions for resolving this issue?