An error occurred while sending email

An old problem has resurfaced in the last month or so.  I cat not send email (using Thunderbird) because the VPN servers that are used with the Private Internet Access service that I use seem to be on some blacklist.  When I turn off the VPN, the email gets sent.

(For history, see Re: 421 Error when sending email)

Here are a number of the error messages:

An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-po-08v.sys.comcast.net resomta-po-08v.sys.comcast.net CSI IP 174.128.xxx.xxx is not permitted to send messages. Please contact support if you feel this is in error..

An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-ch2-08v.sys.comcast.net resomta-ch2-08v.sys.comcast.net CSI IP 194.59.xxx.xxx is not permitted to send messages. Please contact support if you feel this is in error.. (Got this error twice.)

An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-po-06v.sys.comcast.net resomta-po-06v.sys.comcast.net CSI IP 194.59.xxx.xxx is not permitted to send messages. Please contact support if you feel this is in error..

Can someone help me out here?